Gas sensor with separate contaminant detection element

ABSTRACT

A system for detecting an analyte gas in an environment includes a first gas sensor, a first contaminant sensor separate and spaced from the first gas sensor, and electronic circuitry in electrical connection with the first gas sensor to determine if the analyte gas is present based on a response of the first gas sensor. The electronic circuitry is further in electrical connection with the first contaminant sensor to measure a response of the first contaminant sensor over time. The measured response of the first contaminant sensor varies with an amount of one or more contaminants to which the system has been exposed in the environment over time.

BACKGROUND

The following information is provided to assist the reader inunderstanding technologies disclosed below and the environment in whichsuch technologies may typically be used. The terms used herein are notintended to be limited to any particular narrow interpretation unlessclearly stated otherwise in this document. References set forth hereinmay facilitate understanding of the technologies or the backgroundthereof. The disclosure of all references cited herein are incorporatedby reference.

Catalytic or combustible (flammable) gas sensors have been in use formany years to, for example, prevent accidents caused by the explosion ofcombustible or flammable gases. In general, combustible gas sensorsoperate by catalytic oxidation of combustible gases.

The operation of a catalytic combustible gas sensor proceeds throughelectrical detection of the heat of reaction of a combustible gas on theoxidation catalyst, usually through a resistance change. The oxidationcatalysts typically operate in a temperature above 300° C. to catalyzecombustion of an analyte (for example, in the range of 350 to 600° C.temperature range for methane detection). Therefore, the sensor mustsufficiently heat the sensing element through resistive heating. In anumber of combustible gas sensors, the heating and detecting element areone and the same and composed of a platinum alloy because of its largetemperature coefficient of resistance and associated large signal intarget/analyte gas. The heating element may, for example, be a helicalcoil of fine wire or a planar meander formed into a hotplate or othersimilar physical form. The catalyst being heated often is an activemetal catalyst dispersed upon a refractory catalyst substrate or supportstructure. Usually, the active metal is one or more noble metals such aspalladium, platinum, rhodium, silver, and the like and the supportstructure is a refractory metal oxide including, for example, one ormore oxides of aluminum, zirconium, titanium, silicon, cerium, tin,lanthanum and the like. The support structure may or may not have highsurface area (for example, greater than or equal to 75 m²/g). Precursorsfor the support structure and the catalytic metal may, for example, beadhered to the heating element in one step or separate steps using, forexample, thick film or ceramic slurry techniques. A catalytic metal saltprecursor may, for example, be heated to decompose it to the desireddispersed active metal, metal alloy, and/or metal oxide.

As illustrated in FIGS. 1A and 1B, a number of conventional combustiblegas sensors such as illustrated sensor 10 typically include an elementsuch as a platinum heating element wire or coil 20 encased in arefractory (for example, alumina) bead 30, which is impregnated with acatalyst (for example, palladium or platinum) to form an active orsensing element, which is sometimes referred to as a pelement 40,pellistor, detector or sensing element. A detailed discussion ofpelements and catalytic combustible gas sensors which include suchpelements is found in Mosely, P. T. and Tofield, B. C., ed., Solid StateGas Sensors, Adams Hilger Press, Bristol, England (1987). Combustiblegas sensors are also discussed generally in Firth, J. G. et al.,Combustion and Flame 21, 303 (1973) and in Cullis, C. F., and Firth, J.G., Eds., Detection and Measurement of Hazardous Gases, Heinemann,Exeter, 29 (1981).

Bead 30 will react to phenomena other than catalytic oxidation that canchange its output (i.e., anything that changes the energy balance on thebead) and thereby create errors in the measurement of combustible gasconcentration. Among these phenomena are changes in ambient temperature,humidity, and pressure.

To minimize the impact of secondary effects on sensor output, the rateof oxidation of the combustible gas may, for example, be measured interms of the variation in resistance of sensing element or pelement 40relative to a reference resistance embodied in an inactive, compensatingelement or pelement 50. The two resistances may, for example, be part ofa measurement circuit such as a Wheatstone bridge circuit as illustratedin FIG. 1C. The output or the voltage developed across the bridgecircuit when a combustible gas is present provides a measure of theconcentration of the combustible gas. The characteristics ofcompensating pelement 50 are typically matched as closely as possiblewith active or sensing pelement 40. In a number of systems, compensatingpelement 50 may, however, either carry no catalyst or carry aninactivated or poisoned catalyst. In general, changes in properties ofcompensating elements caused by changing ambient conditions are used toadjust or compensate for similar changes in the sensing element.

Catalytic combustible gas sensors are typically used for long periods oftime over which deterioration of the sensing element or the like andmalfunction of circuits may occur. A foreign material or contaminantsuch as an inhibiting material or a poisoning material (that is, amaterial which inhibits or poisons the catalyst of the sensing element)may, for example, be introduced to the sensing element. Contaminants aredeposited upon the surface of an element from the environment. If theelement is heated to a certain temperature, many such materials react(for example, oxidize—either partially or completely) upon the surfaceof the element. Such reaction may result in a species that is morestrongly bound to the surface. An inhibiting, contaminant materialtypically will “burn off” over time, but a poisoning, contaminantmaterial permanently destroys catalytic activity of a sensing element.Inhibiting materials and poisoning materials are sometimes referred toherein collectively as “contaminants” or “contaminant material.” Often,it is difficult to determine such an abnormal operational state orstatus of a combustible gas sensor without knowingly applying a test gasto the combustible gas sensor. In many cases, a detectible concentrationof a combustible gas analyte in the ambient environment is a rareoccurrence. Testing of the operational status of a combustible gassensor typically includes the application of a test gas (for example, agas including a known concentration of the analyte or a simulant thereofto which the combustible gas sensor is similarly responsive) to thesensor. Periodic testing using a combustible gas may, however, bedifficult, time consuming and expensive.

Problems associated with contamination and/or degradation of thecatalyst structures in combustible gas sensors are well known.Sulfur-containing compounds (inhibitors) have been known to target andinhibit the catalyst structures. Filtering techniques are generally usedto prevent their passage into the structure. If they do enter thestructure, they are bound until a sufficient level of heat is applied topromote their release or decomposition. Volatile silicon/organosiliconcompounds (poisons) are also known to cause significant issues withcatalytic structures as they are permanently retained, and eventuallyresult in the total inactivity of the catalyst. Further, high levels ofhydrocarbons can also deposit incomplete and/or secondary byproductssuch as carbon within the structure. Lead compounds, organophosphatesand halogenated hydrocarbons are also known to poison/inhibit catalystsused in combustible gas sensors.

Manufacturers may add a layer of inhibitor/poison (contaminant)absorbing material outside of the supported catalyst of a sensingelement as well as a compensating element. However, exposure to asufficient amount of inhibitor/poison can still render the catalystinactive. Moreover, increasing the mass of the sensing/compensatingelement increases the power requirements of the sensor, which may beundesirable, particularly in the case of a portable or other combustiblegas sensor in which battery power is used.

An inhibited or poisoned sensing element may go undetected by, forexample, high sensitivity bridge and other circuits used in combustiblegas sensors. Users have long reported cases where their catalyticsensors are reading zero (that is, the bridge circuitry is balanced),yet the sensors show little response to gas challenges. A notableexample of this effect occurs when an organosilicon vapor such ashexamethyldisiloxane (HMDS) is introduced to the sensor. The HMDS willindiscriminately diffuse into the sensor housing and surroundings,adsorb onto the surface of the detector and/or compensator, and oxidizeinto a layer of silica (silicon dioxide or SiO₂) or Si_(x)C_(y)O_(z)species. Since both elements are typically operated at similartemperatures, silicone deposition occurs at an equal rate, keeping thebridge in balance. Unfortunately, this renders the elements permanentlyinactive. Indeed, some manufacturers use this poisoning process tomanufacture compensating elements or compensators for combustible gassensors.

A number of methods and systems have been developed in an attempt tosense inhibition/poisoning (contamination) of a catalytic sensingelement with limited success. In general, such methods monitor for achange in properties of the catalytic structure of the gas sensingelement over time. It remains desirable to develop diagnostic systemsand methods for catalytic sensors and structures to detectinhibition/poisoning.

SUMMARY

In one aspect, a system for detecting an analyte gas in an environmentincludes a first gas sensor, a first contaminant sensor separate andspaced from the first gas sensor, and electronic circuitry in electricalconnection with the first gas sensor to determine if the analyte gas ispresent based on a response of the first gas sensor. The electroniccircuitry is further in electrical connection with the first contaminantsensor to measure a response of the first contaminant sensor over time.The measured response of the first contaminant sensor varies with anamount of one or more contaminants to which the system has been exposedin the environment over time. The first gas sensor may, for example, bea first combustible gas sensor.

In a number of embodiments, the first contaminant sensor includes afirst contaminant sensor element separate and spaced from the firstcombustible gas sensor. The first contaminant sensor element includes afirst electrically conductive heating component and a first interfacestructure on the first electrically conductive heating component. Theelectronic circuitry may, for example, be configured to provide energyto the first electrically conductive heating component. In a number ofembodiments, the measured response is a thermodynamic response of thefirst contaminant sensor element which varies with mass of the one ormore contaminants deposited on the first interface structure thereof.

The first combustible gas sensor may, for example, include a firstelement including a first electrically conductive heating element, afirst support structure on the first electrically conductive heatingelement and a first catalyst supported on the first support structure.The electronic circuitry may, for example, be configured to provideenergy to the first electrically conductive heating element to heat thefirst element to at least a first temperature at which the firstcatalyst catalyzes combustion of the analyte gas and to determine if theanalyte gas is present based on the response of the first combustiblegas sensor while the first element is heated to at least the firsttemperature.

In a number of embodiments, the first contaminant sensor furtherincludes a second contaminant sensor element. The second contaminantsensor element may include a second electrically conductive heatingcomponent and a second interface structure on the second electricallyconductive heating component. The electronic circuitry may, for example,be configured to operate the second contaminant sensor element as acompensating element for at least the first contaminant sensor elementto compensate for ambient conditions. In a number of embodiments, thesecond contaminant sensor element is treated to be generally insensitiveto at least one of the one or more contaminants. The second contaminantsensor element may, for example, be treated with a predetermined amountof an oxidized organosilicon compound.

In a number of embodiments, the first interface structure is selected toadsorb at least one of the one or more contaminants that undergooxidation upon heating. The first interface structure may, for example,include an oxide. In a number of embodiments, the first interfacestructure includes a silicon oxide or a metal oxide. The first interfacestructure may, for example, have a surface area of at least 75 m²/g. Thefirst interface structure may, for example, include a refractory metaloxide. The first interface structure may, for example, include aluminumoxide, tin oxide, zinc oxide or copper oxide.

In a number of embodiments, the first contaminant sensor elementincludes no metal catalyst. The first contaminant sensor element may,for example, consists essentially of the first electrically conductiveheating component and the first interface structure, which consistsessentially of an oxide.

In a number of embodiments, the system further includes a first filterpathway between the first gas sensor and the environment. The firstfilter pathway has a first capacity to remove at least one of the one ormore contaminants. The system further includes a second filter pathwaybetween the first contaminant sensor and the environment. The secondfilter pathway has a second capacity to remove at least one of the oneor more contaminants. The second capacity is less than the firstcapacity. In a number of embodiments, the first capacity includes afirst adsorbent filtration capacity and the second capacity includes asecond adsorbent filtration capacity, less than the first adsorbentfiltration capacity.

In a number of embodiments, the system includes a first filter pathwaybetween the first element of the first combustible gas sensor and theenvironment, which has a first capacity to remove at least one of theone or more contaminants, and a second filter pathway between the firstcontaminant sensor element and the environment, which has a secondcapacity to remove at least one of the one or more contaminants, whereinthe second capacity is less than the first capacity. As set forth above,the first capacity may include a first adsorbent filtration capacity,and the second capacity may include a second adsorbent filtrationcapacity, less than the first adsorbent filtration capacity. In a numberof embodiments, the second adsorbent filtration capacity is zero.

In a number of embodiments, the first element of a first combustible gassensor hereof is low-thermal-mass element. The first element of thefirst combustible gas sensor may, for example, a thermal time constantless than 8 seconds or less than 1 second. The first element of thefirst combustible gas sensor may, for example, be a MEMS element. Thefirst element of the first combustible gas sensor may, for example, be alow-thermal-mass pelement.

In a number of embodiments, the first contaminant sensor element islow-thermal mass element. The first contaminant sensor element may, forexample, have a thermal time constant less than 8 seconds of less than 6second. In a number of embodiments, the first contaminant sensor elementis a low-thermal-mass pelement.

In a number of embodiments, a pulse is applied to the first contaminantsensor element in which energy to the first contaminant sensor elementis increased or decreased to induce the measured response from the firstcontaminant sensor element. The electronic circuitry may, for example,be configured to analyze the measured response.

In a number of embodiments, a temperature of the second contaminantsensor element is maintained below a temperature at which at least oneor the one or more contaminants is oxidized on the second interfacestructure. The temperature of the second contaminant sensor element may,for example, be maintained below 150° C. or below 90° C.

The temperature of the first contaminant sensor element may, forexample, be increased via an applied pulse to induce joule heating andfor sufficient time to raise the temperature of the first contaminantsensor element. In a number of embodiments, energy is decreased via anapplied pulse from a temperature of at least the first temperature suchthat convective heat transfer between the first interface structure andsurrounding gas ceases, and for sufficient time so that the temperatureof the first contaminant sensor element decreases below the temperatureat which joule heating of the first contaminant sensor element occurs.

In a number of embodiments, the electronic circuitry is configured toapply a plurality of pulses to the first contaminant sensor element overtime in which energy to the first element is increased or decreased toinduce the measured response from the first contaminant sensor elementin each of the plurality of pulses. The electronic circuitry may, forexample, be configured to analyze one or more of the measured responses.

In a number of embodiments, the electronic circuitry is configured toadjust an output associated with a response of the combustible gassensor based upon the measured response of the first contaminant sensor.

In another aspect, a method for detecting an analyte gas in anenvironment includes providing a first gas sensor, providing a firstcontaminant sensor separate and spaced from the first gas sensor,providing electronic circuitry in electrical connection with the firstgas sensor and with the first contaminant sensor, measuring a responseof the first gas sensor to determine via the electronic circuitry if theanalyte gas is present, and measuring a response of the firstcontaminant sensor to determine via the electronic circuitry if thesystem has been exposed to one or more contaminants. The measuredresponse of the first contaminant sensor varies with an amount of one ormore contaminants to which the system has been exposed in theenvironment over time.

In a number of embodiments, the first gas sensor is a first combustiblegas sensor. The first contaminant sensor may, for example, include afirst contaminant sensor element separate and spaced from the firstcombustible gas sensor. The first contaminant sensor element includes afirst electrically conductive heating component and a first interfacestructure on the first electrically conductive heating component. Theelectronic circuitry is configured to provide energy to the firstelectrically conductive heating component. The measured response of thefirst contaminant sensor is a thermodynamic response of the firstcontaminant sensor element which varies with mass of the one or morecontaminants deposited on the first interface structure thereof.

In a number of embodiments, the first combustible gas sensor includes afirst element including a first electrically conductive heating element,a first support structure on the first electrically conductive heatingelement and a first catalyst supported on the first support structure.The electronic circuitry may, for example, be configured to provideenergy to the first electrically conductive heating element to heat thefirst element to at least a first temperature at which the firstcatalyst catalyzes combustion of the analyte gas and to determine if theanalyte gas is present based on the response of the first combustiblegas sensor while the first element is heated to at least the firsttemperature.

In a number of embodiments, the first contaminant sensor furtherincludes a second contaminant sensor element. The second contaminantsensor element may, for example, include a second electricallyconductive heating component and a second interface structure on thesecond heating electrically conductive heating component. The method mayfurther include operating the second contaminant sensor element via theelectronic circuitry as a compensating element for at least the firstcontaminant sensor element to compensate for ambient conditions.

In a further aspect, a system includes electronic circuitry comprising acontrol system, a primary combustible gas sensor in electricalconnection with the electronic circuitry to determine if an analyte gasis present based on a response of the primary combustible gas sensor anda trigger combustible gas sensor in electrical connection with theelectronic circuitry to determine if the analyte gas is present based ona response of the trigger combustible gas sensor. The electroniccircuitry is configured to operate the trigger combustible gas sensor todetect a value of a response at or above a threshold value. The primarycombustible gas sensor is activated from a low-power state upon thethreshold value being detected by the trigger combustible gas sensor.The system further includes a first contaminant sensor in electricalconnection with the electronic circuitry, which is positioned separateand spaced from the primary combustible gas sensor and from the triggercombustible gas sensor. The electronic circuitry is further configuredto measure a response of the first contaminant sensor over time. Themeasured response of the first contaminant sensor varies with an amountof one or more contaminants to which the system has been exposed in theenvironment over time.

In a number of embodiments, the primary combustible gas sensor includesa first primary element in operative connection with the electroniccircuitry and including a first primary support structure, a firstprimary catalyst supported on the first primary support structure and afirst primary heating element in operative connection with the firstprimary support structure. The trigger combustible gas sensor may, forexample, include a first trigger element of low-thermal-mass inoperative connection with the electronic circuitry. The first triggerelement may, for example, include a first trigger heating element, afirst trigger support structure and a first trigger catalyst supportedon the first trigger support structure.

In a number of embodiments, the first contaminant sensor includes afirst contaminant sensor element separate and spaced from the primarycombustible gas sensor and the trigger combustible gas sensor. The firstcontaminant sensing element may, for example, include a firstelectrically conductive heating component and a first interfacestructure on the first electrically conductive heating component. Theelectronic circuitry may, for example, be configured to provide energyto the first electrically conductive heating component.

In a number of embodiments, the system further includes a first filterpathway between the trigger combustible gas sensor and the environment.The first filter pathway may, for example, have a first capacity toremove at least one of the one or more contaminants. The system mayfurther include a second filter pathway between the primary combustiblegas sensor and the environment. The second filter pathway may, forexample, have a second capacity to remove at least one of the one ormore contaminants. The system may further include a third filter pathwayhaving a third capacity between the first contaminant sensor and theenvironment. The third capacity is less than the first capacity and lessthan the second capacity. In a number of embodiments, the secondcapacity is less than the first capacity. The first capacity may, forexample, include a first adsorbent filtration capacity. The secondcapacity may, for example, include a second adsorbent filtrationcapacity. The third capacity may, for example, include a third adsorbentfiltration capacity. In a number of embodiments, the third adsorbentfiltration capacity is zero.

The present devices, systems, and methods, along with the attributes andattendant advantages thereof, will best be appreciated and understood inview of the following detailed description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates an embodiment of a currently available combustiblegas sensor.

FIG. 1B illustrates an enlarged view of the active sensing element,pelement or detector of the combustible gas sensor of FIG. 1A.

FIG. 1C illustrates an embodiment of circuitry for the combustible gassensor of FIG. 1A.

FIG. 2A illustrates a perspective view of an embodiment of a detectorassembly wherein a sensing element is supported by a conductivesupporting wire.

FIG. 2B illustrates a perspective view of the detector assembly of FIG.2A including a ceramic bead (upon which a catalyst is supported) formedover the sensing element wire.

FIG. 2C illustrates another perspective view (generally opposite that ofFIG. 2B) of the detector assembly of FIG. 2A.

FIG. 3A illustrates schematically a cross-sectional view of anembodiment of a low-thermal mass, MEMS hotplate combustible gas sensorsuitable for use herein.

FIG. 3B illustrates a perspective view of the low-thermal-masscombustible gas sensor of FIG. 3A in operative connection with a printedcircuit board.

FIG. 4A illustrates schematically a combustible gas sensor device orinstrument including two detector or sensor assemblies as illustrated inFIGS. 2A through 2C for analyte detection and a third, separate detectoror sensor assembly of FIGS. 2A through 2C for contaminant detection inelectrical connection with control and measurement circuitry.

FIG. 4B illustrates an embodiment of a simulated bridge circuit for usein the circuitry of the sensor of FIG. 4A.

FIG. 5A illustrates change in the 200 ms dynamic response over thecourse of 44 ppm-h HMDS poisoning.

FIG. 5B illustrates the contaminant schedule for the experiment of FIG.5A wherein the per step dose is shown by the solid line and thecumulative dose is shown by the dotted line.

FIG. 6 illustrates predicted HMDS exposure using a balanced model withspline coefficient fits, with actual measured exposure shown on theordinate.

FIG. 7 illustrates methane sensitivity as a function of time incontaminant exposure for sensors including filter elements or componentsfor contaminants including HDMS which were tested in 15 ppm HMDS atstandard run temperature

FIG. 8 illustrates a light-off curve for hexamethyldisiloxane (HMDS) viasensitivity loss in methane of a catalytically active analyte sensingelement as a function of exposure temperature in HMDS.

FIG. 9 illustrates response of a contaminant sensing element includingan oxide interface structure to 50 ppm-hour HMDS as a function of aperiod of time the contaminant sensing element remains unpowered priorto application of a “loading pulse” thereto in the form of a pulse ofenergy.

FIG. 10A illustrates schematically a combustible gas sensor device orinstrument including a MEMS hotplate sensor as illustrated in FIGS. 3Aand 3B which is operable as a “sniffer sensor”, two low-thermal-masspelements as illustrated in FIGS. 2A through 2C which are operable as aprimary combustible gas sensor, and a third, separate low-thermal-masspelement as illustrated FIGS. 2A through 2C which is operable forcontaminant detection, all of which are in electrical connection withcontrol and measurement circuitry via a printed circuit board or PCB400.

FIG. 10B illustrates a perspective view of a portion of the device orinstrument of FIG. 10A without filters in place.

FIG. 11 illustrates schematically another embodiment of a combustiblegas sensor device or instrument including a MEMS hotplate sensor whichis operable as a “sniffer sensor”, a pelement assembly as illustrated inFIGS. 2A through 2C which is operable as a triggerable primarycombustible gas sensor, two separate pelement assemblies as illustratedin FIGS. 2A through 2C which are operable as a contaminant sensor, allof which are electrical connection with control and measurementcircuitry via a PCB 400.

FIG. 12 illustrates schematically another embodiment of a combustiblegas sensor device or instrument including a MEMS hotplate sensor asillustrated in FIGS. 3A and 3B which is operable as a “sniffer sensor”,a low-thermal-mass pelement which is operable as a sensor for analytedetection, a second, separate low-thermal-mass pelement which isoperable as a first contaminant sensor in connection with a thirdlow-thermal-mass pelement used for compensation and a fourthlow-thermal-mass pelement which is operable as a second contaminantsensor in connection with the third, compensating pelement, wherein thefirst contaminant sensor is correlated with the MEMS hotplate sensor,and the second contaminant sensor is correlated with the analyte sensingpelement(s).

DETAILED DESCRIPTION

It will be readily understood that the components of the embodiments, asgenerally described and illustrated in the figures herein, may bearranged and designed in a wide variety of different configurations inaddition to the described example embodiments. Thus, the following moredetailed description of the example embodiments, as represented in thefigures, is not intended to limit the scope of the embodiments, asclaimed, but is merely representative of example embodiments.

Reference throughout this specification to “one embodiment” or “anembodiment” (or the like) means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment. Thus, the appearance of the phrases “in oneembodiment” or “in an embodiment” or the like in various placesthroughout this specification are not necessarily all referring to thesame embodiment.

Furthermore, described features, structures, or characteristics may becombined in any suitable manner in one or more embodiments. In thefollowing description, numerous specific details are provided to give athorough understanding of embodiments. One skilled in the relevant artwill recognize, however, that the various embodiments can be practicedwithout one or more of the specific details, or with other methods,components, materials, etcetera. In other instances, well knownstructures, materials, or operations are not shown or described indetail to avoid obfuscation.

As used herein and in the appended claims, the singular forms “a,” “an”,and “the” include plural references unless the context clearly dictatesotherwise. Thus, for example, reference to “a sensing element” includesa plurality of such sensing element and equivalents thereof known tothose skilled in the art, and so forth, and reference to “the sensingelement” is a reference to one or more such sensing elements andequivalents thereof known to those skilled in the art, and so forth.

The terms “electronic circuitry”, “circuitry” or “circuit,” as usedherein includes, but is not limited to, hardware, firmware, software orcombinations of each to perform a function(s) or an action(s). Forexample, based on a desired feature or need. a circuit may include asoftware controlled microprocessor, discrete logic such as anapplication specific integrated circuit (ASIC), or other programmedlogic device. A circuit may also be fully embodied as software. As usedherein, “circuit” is considered synonymous with “logic.” The term“logic”, as used herein includes, but is not limited to, hardware,firmware, software or combinations of each to perform a function(s) oran action(s), or to cause a function or action from another component.For example, based on a desired application or need, logic may include asoftware controlled microprocessor, discrete logic such as anapplication specific integrated circuit (ASIC), or other programmedlogic device. Logic may also be fully embodied as software.

The term “processor,” as used herein includes, but is not limited to,one or more of virtually any number of processor systems or stand-aloneprocessors, such as microprocessors, microcontrollers, centralprocessing units (CPUs), and digital signal processors (DSPs), in anycombination. The processor may be associated with various other circuitsthat support operation of the processor, such as random access memory(RAM), read-only memory (ROM), programmable read-only memory (PROM),erasable programmable read only memory (EPROM), clocks, decoders, memorycontrollers, or interrupt controllers, etc. These support circuits maybe internal or external to the processor or its associated electronicpackaging. The support circuits are in operative communication with theprocessor. The support circuits are not necessarily shown separate fromthe processor in block diagrams or other drawings.

The term “controller,” as used herein includes, but is not limited to,any circuit or device that coordinates and controls the operation of oneor more input and/or output devices. A controller may, for example,include a device having one or more processors, microprocessors, orcentral processing units capable of being programmed to performfunctions.

The term “logic,” as used herein includes, but is not limited to.hardware, firmware, software or combinations thereof to perform afunction(s) or an action(s), or to cause a function or action fromanother element or component. Based on a certain application or need,logic may, for example, include a software controlled microprocess,discrete logic such as an application specific integrated circuit(ASIC), or other programmed logic device. Logic may also be fullyembodied as software. As used herein, the term “logic” is consideredsynonymous with the term “circuit.”

The term “software,” as used herein includes, but is not limited to, oneor more computer readable or executable instructions that cause acomputer or other electronic device to perform functions, actions, orbehave in a desired manner. The instructions may be embodied in variousforms such as routines, algorithms, modules or programs includingseparate applications or code from dynamically linked libraries.Software may also be implemented in various forms such as a stand-aloneprogram, a function call, a servlet, an applet, instructions stored in amemory, part of an operating system or other type of executableinstructions. It will be appreciated by one of ordinary skill in the artthat the form of software is dependent on, for example, requirements ofa desired application, the environment it runs on, or the desires of adesigner/programmer or the like.

In a number of representative embodiments hereof, one or morecontaminant sensors hereof are combined with or incorporated with one ormore combustible gas sensors. However, the contaminant sensors hereofare beneficial for use with any sensor or multi-sensor system having anelement or component which is sensitive to mass deposition of one ormore contaminants thereon as discussed further below. In general, such acontaminant or contaminants is/are compositions other than the analyteor target composition(s) for the sensor or sensor system. Such acontaminant may, for example, degrade the performance of a sensor orsensor system in one or more manners. In a number of embodiments, theelements or components of a sensor or sensor system upon which one ormore contaminants may deposit are heated elements or components (forexample, a sensing element, an energy source etc.) which are heated to atemperature at which one or more adsorbed/chemisorbed contaminants reactto bind with a surface of the element or component. In otherembodiments, such elements or components include, for example, a filtermaterial upon which one or more contaminant may deposit. One or morecontaminants may, for example, be an interferent for the sensor (thatis, a composition to which the sensor exhibits cross-sensitivity), aninhibitor, a poison, etc. Representative sensors with which thecontaminant sensors hereof may be used in combination include, but arenot limited to, combustible gas sensors, metal oxide sensors (MOS),solid state oxygen sensors, photoionization detectors (PIDs), andelectrochemical sensors. One or more contaminant sensors hereof may, forexample, be positioned within a common housing with one or more suchsensors.

In a number of representative embodiments hereof, devices, systems andmethods of determining the well-being or operational status of a one ormore components (for example, a sensing element including a catalyticstructure and/or a filter) in a sensor such as a combustible gas sensorvia a separate, contaminant sensor are set forth. The devices, systemsand methods hereof do not require the use or application of a test gasor any other gas to the sensor in determining contaminant exposure. Atest gas is a gas which includes a non-zero known concentration of theanalyte (or target) gas or a simulant thereof. In the devices, systemsand methods hereof, a contaminant sensor (including, for example, acontaminant sensing element or detector), which is physically separatefrom any analyte or target gas sensing element or any compensatingelement, is provided. Contaminant sensing elements hereof may, forexample, include a heating component or element (typically a conductivecomponent or element) and an interface structure disposed on the heatingcomponent or element. Contaminants are deposited/adsorbed/chemisorbedupon the surface of the interface structure, and certain contaminants(for example, sulfur compounds and silicon/organosilicon compounds) maybecome strongly bound thereto upon heating/reaction. In a number ofembodiments, the interface structure includes an oxide, which may be arefractory or heat-resistant material (for example, a refractory metaloxide). In a number of embodiments, the interface structure has asurface area of at least 75 m²/g, or a surface area of at least 150m²/g.

During the application of low voltages (for example, 0V-0.25V), to aheating element wire or coil such as coil 20 (that is, a heating elementor component), the element resistance remains consistent. In such avoltage range, resistive changes are predominantly governed by ambienttemperature fluctuations. The principles employed in this regime arewell known and are used, for example, in resistive thermometers. In thatregard, the platinum resistance thermometer is a versatile instrumentfor temperature measurement in the range from approximately −200° C. to+1000° C. One may, for example, use the simplified Callendar—Van Dusenequation to determine the temperature dependent resistance as follows:

R _(t) =R ₀[1+α(t−t ₀)]

wherein R_(t) is the resistance of the element at temperature t, R₀ isthe resistance at a standard temperature to, and a is the temperaturecoefficient of resistance. The above principle may, for example, be usedas described in U.S. Pat. No. 8,826,721, the disclosure of which isincorporated herein by reference, to operate an element of a combustiblegas sensor (which may be a catalytically active sensing element or acatalytically inactive element) in a low power (voltage),low-temperature mode in which the element is able to function as acompensating element or compensator.

The application of higher voltages (for example, >0.5V) will cause theheating element or component to increase in temperature, and thus inresistance. This effect is known as Joule's first law or the Joule-Lenzlaw. Joule heating, also known as ohmic heating or resistive heating, isthe process by which the passage of an electric current through aconductor releases heat. In the case of, for example, an analyte elementincluding a catalyst support structure or a contaminant sensing elementhereof including an interface structure, the heat transfer from theheating element/component will eventually reach an equilibrium as theheat will conduct from the heating element to the structure overlayingthe heating element (including, for example, an oxide or refractorymaterial and any catalyst supported thereon) and then via fluidicconvection through the surrounding gases. Thermal equilibrium willremain balanced until (a) the ambient temperature changes; (b) themakeup of the surrounding gas mixture is altered, or (c) the transfer ofheat between the wire and the mass of the element changes (as a resultof a mass or density change). These effects are all competing andinteracting effects.

In the case of a combustible gas sensor, a heating element such asheating element 20 of FIG. 1B (for example, a conductive wire, coil orsurface) is used to sufficiently raise the structure of the element(including the support structure and catalyst) to a temperature topromote the catalytic reaction of the analyte or target gas. As usedherein with respect to an element hereof (that is, an analyte sensingelement or analyte element, a compensating element or a contaminationsensing element), temperature refers to an average temperature over thevolume of the element. Heating elements or components have generallybeen made from coils, and over time smaller diameter wires have beenused to reduce the power consumption of the element.

The use of conductive elements or components such as wires havingrelatively small diameter in element for combustible gas sensors is, forexample, disclosed in U.S. Pat. No. 8,826,721 and U.S. PatentApplication Publication No. 2018/0128763, the disclosure of which isincorporated herein by reference. In that regard, FIGS. 2A through 2Cillustrate a representative embodiment of a detector/element assembly110 which may, for example, be used in a combustible gas sensor. Elementassembly 110 includes a base 120 to which two electrically conductivecontact members 130 (extending members or posts in the illustratedembodiment) are attached. A sensing conductive element or heatingelement 140 is connected between contact members 130, wherein each endof conductive elements 140 is connected to or anchored to one of contactmembers 130. In the illustrated embodiment, conductive element 140includes an intermediate section including a coiled section 142 thatcan, for example, be located approximately centrally between the ends ofconductive element 140. Wires and/or other conductive elements forheating elements or components are selected to have a favorabletemperature coefficient for sensing applications and are generally aprecious metal or alloy.

Element assembly 110 further includes two support members 150 (extendingmembers or posts in the illustrated embodiment) connected to base 120.In the illustrated embodiment, a support member or element 160 in theform of, for example, a wire, a ribbon, a rod or other suitable supportstructure or material extends between support members or posts 150. Base120, contact members 130 and support members 150 can, for example, beformed of a metal such as KOVAR® (a nickel-cobalt ferrous alloy designedto be compatible with the thermal expansion characteristics ofborosilicate glass) available from Carpenter Technology Corporation ofReading, Pa. Contact members 130 and support members 150 can, forexample, be sealed to base 120 using a glass such as borosilicate glassto provide electrical isolation.

Using a strong yet relatively thin support element 160 anchored,connected or attached at each end thereof (for example, anchored at twosupport members or posts 150) prevents bead movement in all threedimensions while limiting heat loss. In the illustrated embodiment ofFIGS. 2A through 2C, support element 160 passes through and contacts oneof the coils of coiled section 142. Contact between support element 150and conductive element 140 is thus minimal. As described below, supportelement 150 need not contact conductive element 140 to provide supporttherefor, but can contact or pass through a catalyst support member orstructure 170 encompassing conductive element 140.

A balance may, for example, be established between the tensile strengthand the thermal conductivity to achieve an effective result for supportelement 150. In general, a quotient or ratio calculated by dividing thetensile strength in units of pounds per square inch of psi by thethermal conductivity in units of watts/cm/° C. may, for example, be atleast 250,000, at least 400,000 or even at least 500,000. For example, asupport element in the form of a wire made from an alloy of platinum andtungsten may have a tensile strength of 250,000 psi and a thermalconductivity of 0.5 watts/cm/° C., resulting in a quotient of 500,000.For support elements having a higher tensile strength, a higher thermalconductivity may be acceptable since support elements of smaller averagediameter (or average cross-sectional area) can be used (resulting inless mass to conduct heat away from the sensing element). Moreover,reducing the size/volume of the element reduces the effect of ambienthumidity and pressure changes on the sensor. For example, in the case ofa tungsten support element having a tensile strength of 600,000 psi anda thermal conductivity of 1.27 watts/cm/° C., a smaller average diametersupport element can be used to achieve a similar result to that achievedwith the platinum-tungsten alloy support element described above.Alternatively, one could also choose a support element of an alloy ofplatinum with 20% iridium having a larger average diameter. Such aplatinum-iridium alloy has a tensile strength of 120,000 psi and athermal conductivity of 0.18 watts/cm/° C. Metal support elements ormetal alloy elements having the above-described properties can be usedto maximize strength/support while minimizing heat loss.

In that regard, in several embodiments, support element 160 exhibitsrelatively high strength (for example, having a tensile strength of atleast 100,000 psi, at least 250,000 psi, or even at least 400,000 psi)as well as low thermal conductivity (for example, having a thermalconductivity less than 1.5 less watts/cm/° C., less than 0.5 watts/cm/°C., no greater than 0.25 watts/cm/° C., or even no greater than 0.10watts/cm/° C.) to provide a quotient as described above. In a number ofembodiments, the average diameter of support element 160 (in the case ofa support element of a generally circular cross-section) is in the rangeof approximately 0.0005 (12.7 μm) to 0.0025 inches (63.5 μm). In thecase of support elements having a noncircular cross-section, the averagecross-sectional area can, for example, be in the range of the averagecross-sectional area of an element of generally circular cross-sectionhaving an average diameter in the range of approximately 0.0005 to0.0025 inches. References herein to elements having a certain averagediameter are also references to elements having a generally noncircularcross-section, but having an average cross-sectional area equivalent tothe average cross-sectional area provided by the stated averagediameter. In several representative studies, an in-molded wire was usedas support element 160. In several such embodiments, a platinum-tungstenalloy support element 160 having an average diameter of approximately(that is, within 10% of) 0.001 inches (63.5 μm) provided a robustsupport and did not result in measurable additional power required tooperate sensing element 140. Alloys of tungsten, nickel, molybdenum ortitanium with, for example, platinum, palladium or rhodium can, forexample, be used in support element 160.

As illustrated in FIG. 2B, catalyst support structure 170 (for example,a ceramic bead in a number of embodiments) can be formed on coil section120 of sensing conductive element 140 to support a catalyst and form asensing element/pelement. In forming catalyst support structure 170 as arefractory material such as a ceramic bead, an aluminum oxide suspensionmay, for example, be fired onto coiled section 142. The resultantcatalyst support structure/ceramic bead 170 may be impregnated with acatalyst. Although a bare wire comprising a catalytic material (such asplatinum) can be used as a sensing element in certain embodiments of acombustible gas sensor, a catalyst support structure 170 (such as aceramic bead) provides increased surface area for one or more catalystspecies.

In the embodiment illustrated in FIGS. 2A through 2C, catalyst supportstructure 170 is formed over (to encompass) conductive element 140 andsupport element 160. Support element 160 need not contact conductiveelement 140 to provide support therefor. For example, support element160 can pass through or contact support structure 170 without contactingconductive element 140 and indirectly provide support for conductiveelement 140. To provide support for conductive element 140 in threedimensions, support element 160 preferably passes through catalystsupport structure 170.

The support assembly, including, for example, support member 150 andsupport element 160, enables the use of a sensing element 140 having arelatively small average diameter. For example, a wiring having anaverage diameter no greater than approximately 20 μm of 10 μm may beused. Such a small average diameter wire (with a corresponding higherper unit length resistance than larger diameter wires) lends itself wellto reducing the required operating current (which is very desirable inportable applications), and thus the required power levels. In a numberof embodiments, the support members or catalyst support members hereofhave a volume less than 6.5×10⁷ μm³, less than 4.46×10⁷ μm³, or eventhan 1.4×10⁷ μm³.

As known in the art, a heating element in the form of a wire or wirecoil may be dipped it into an aqueous suspension of a precursor of arefractory. The precursor may then be converted into the refractorymaterial by heating (for example, by the passage of an electricalheating current through the heating element). The dipping process isusually repeated to build up a support structure of the desiredsize/average diameter around the heating element. In forming acatalytically active element, a solution or dispersion of a catalyst maythen be applied to the outer surface of the support structure.

Low thermal time constants associated with low thermal mass sensors suchas the low-thermal-mass pelements described above assist in providingquick response times, reducing the time an element may be unavailablefor use in a detection mode and decrease power requirements.Low-thermal-mass elements hereof may, for example, have a thermal timeconstant of 8 second or less, 6 seconds or less, 1 second or less, 0.5seconds or less or 0.250 second or less. A low thermal mass/low thermaltime constant sensor may, for example, be a pelement of low thermal massas described above or a microelectronic mechanical systems (MEMS)element to provide a thermal time constant. As used herein the thermaltime constant of an element is defined as the time required to change63.2% of the total difference between its initial and final temperaturewhen subjected to a step function change in drive power, under zeropower initial conditions. MEMS elements typically have a lower thermaltime constant than low-thermal-mass pelements. MEMS elements may, forexample, have thermal time constants of 1 second or less, 0.5 seconds orless or 0.250 second or less.

Oxidation catalysts formed onto a helical wire heater as described aboveare typically referred to as pelements, while those formed ontohotplates (whether MEMS hotplates or conventional, larger hotplates) aresometimes known by the substrate. Oxidative catalysts formed on MEMSheating elements are sometimes referred to as MEMS pellistors. As usedherein, the term “MEMS pellistor” or “MEMS element” refers to a sensorcomponent with dimensions less than 1 mm that is manufactured viamicrofabrication techniques. In a number of representative embodiments,sensing elements formed as MEMS pellistors hereof may be manufacturedwith a thick film catalyst, powered to an operating temperature byresistive heating and are used to detect combustible gases. In a numberof representative embodiments, the thickness and diameter for a MEMScatalyst film is approximately 15 microns and approximately 650 microns,respectively.

FIG. 3A illustrates a cutaway view of an embodiment of a MEMS ormicro-hotplate sensor 200 hereof, which includes a housing 202 having agas inlet 210. A screen or cap 220, which may include or function as afilter 230, may, for example, be placed in connection with inlet 210.The energy (current and voltage) used in MEMS micro-hotplate sensor 200may, for example, be sufficiently low to provide intrinsic safety suchthat a flashback arrestor, as known in the combustible gas detectorarts, may not be necessary. As described above, flashback arrestors (forexample, porous frits) allow ambient gases to pass into a housing butprevent ignition of combustible/flammable gas in the surroundingenvironment by hot elements within the housing. One or more heatingelements or hotplates 240 may, for example, be used to heat an oxidativelayer 252 (which may, for example, be an oxidative catalyst layer) of afirst MEMS element or pellistor 250 to a first operating temperature. Ina number of embodiments, a second MEMS element or second pellistor 250′may be included within MEMS hotplate trigger sensor 200 to be heated toa second operating temperature.

In a number of embodiments, first MEMS element 250 may be operated as asensing or detecting element and second MEMS element 250′ may beoperated as a compensating element as known in the combustible gassensor arts. In other embodiments, as further described below, thefunction of MEMS elements 250 and 250′ which each include an activecatalyst layer may be switched between analyte sensing and compensatingby altering the mode of operation thereof.

Typically, compensating elements include a deactivated catalyst layer orother deactivation layer which destroys the activity of the compensatingelement to oxidize combustible analyte gases. Such inactive compensatingelements are typically operated at the same temperature of the analyteelement. As described in U.S. Pat. No. 8,826,721, the operation of aparticular element as a sensing element or a compensating element may becontrolled by controlling the operating temperature thereof. If theoperating temperature of an element is maintained at or above atemperature at which gas will combust at the surface thereof, it may beoperated as a sensing element. If the operating temperature of anelement is maintained below a temperature at which gas will combust atthe surface thereof, it may be operated as a compensating element. Thetemperature at which gas will combust at the surface of an elementdepends upon the composition of that surface. Surfaces including acatalytic material will typically cause combustion at a temperature (acatalytic light-off temperature) lower than a surface not including acatalytic material. An element including a catalytic material may bealternated between use as a sensing element and use as a compensatingelement through control of the operating temperature thereof (that is,between a higher temperature operational/sensing mode and a lowertemperature/compensating mode).

If operated solely as a MEMS compensator element 250′ may, for example,include an inactive layer 252′ which may be heated by one or moreheating elements or hotplates 240′. In this case, the second operatingtemperature may be maintained at a temperature lower than thetemperature required to cause combustion at a surface thereof in theabsence of a catalyst. Alternatively layer 252′ may include an activecatalyst and be operated at a sufficiently low temperature to preventcatalytic oxidation of combustible gas at the surface thereof. Thesecond temperature may, for example, be ambient temperature.

MEMS hotplate sensor 200 may, for example, mounted on a printed circuitboard or PCB 280. The two resistances of the element 250 and element250′ may, for example, be part of a measurement circuit such as aWheatstone bridge circuit as illustrated in FIG. 1C or a simulatedWheatstone bridge circuit. A representative example of a MEMS hotplatesensor suitable for use herein is an SGX MP7217 hotplate sensor orpellistor available from SGX Sensortech, SA of Corcelles-Coromondreche,Switzerland. Such a MEMS hotplate sensor is disclosed, for example, inU.S. Pat. No. 9,228,967, the disclosure of which is incorporated hereinby reference. MEMS technology, thin/thick film system technology, orother suitable micro- or nanotechnology may be used in forminglow-thermal-mass elements for use herein. See, for example, U.S. Pat.Nos. 5,599,584 and/or 6,705,152, the disclosures of which areincorporate herein by reference.

As described above, the operation of a catalytic combustible gas sensormay proceed through electrical detection of the heat of reaction of acombustible gas on the oxidation catalyst (for example, through aresistance change via a Wheatstone bridge). The oxidation catalysts may,for example, operate in the temperature range of 350-600° C. for methanedetection. Among common hydrocarbons, methane requires the highesttemperature for combustion, hydrogen requires low temperatures, andlarger alkanes fall in between, with longer to shorter carbon chainrequiring lower to higher light-off temperatures.

The analyte elements, compensating elements and/or contaminant sensingelements hereof may be operated in either a comparative/continuous modeor in a dynamic mode. The amount of contaminant deposited upon acontaminant sensing element hereof may be relatable to, or correlatedwith, an amount or dosage (that is, exposure of a certain concentrationover a certain period of time—for example, in the units of ppm-hour) ofone or more contaminants experienced by a device or system hereof(and/or one or more components thereof) over time.

In a number of representative embodiments, comparative methods ormeasurements are used in determining deposition of contaminants on acontaminant sensing element. One skilled in the art appreciates that anumber of different variables related to or relatable to a change inthermal properties of a contaminant sensing element hereof associatedwith a change in mass of the element may be used. Changes in one or moresuch variables are, for example, related to or indicative of a change inmass resulting from the presence of a contaminant on the interfacestructure of the contaminant sensing element. In a number ofembodiments, changes in an electrical property (for example, resistance)of a conductive heating element of a contaminant sensing elementassociated with changes in the thermal properties of the contaminantsensing element are monitored. A variable such as voltage, current orresistance may, for example, be measured depending upon the manner inwhich the electrical circuitry of a sensor or instrument hereof iscontrolled. For example, voltage or current in an electronic circuit canbe measured and related to a change in resistance of a contaminantsensing element. Alternatively, electronic circuitry of a sensor may bedriven to maintain resistance of the contaminant sensing elementrelatively constant and a voltage or a current may be measured.

In the case of a comparative or continuous mode of operation, an elementmay, for example, be operated at a generally constant voltage, aconstant current or a constant resistance (and thereby at a constanttemperature) as described above during a particular mode of operation.To operate in a constant voltage, a constant current or a constantresistance mode, closed loop control is used.

In an open-loop control methodology wherein temperature varies over theinterrogation period, one may use a variety of dynamic, pulsed, ormodulated operations in the devices, systems and methods hereof. In a“dynamic-mode” or “dynamic interrogation mode” operational mode hereof,an element is, for example, briefly energized or de-energized via achange in the electric current flowing therethrough. The length of timeof such dynamic interrogation pulses or changes may, for example, bevery short in the case of low-thermal-mass elements. Once again, theelements hereof may (but need not) have a low thermal mass as describedabove. During an individual energy change or pulse, an element hereofexperiences transitions through different thermal states as thetemperature thereof changes over time. In a number of embodimentshereof, an interrogation method may be based on the observation of thenon-linear electrical response in the electronic circuitry hereof, ofwhich a catalyst support structure (and the catalyst supported thereon)or an interface structure is a part, as the non-linear thermodynamicaction in the element transitions from one thermal state (andtemperature) to another. A support structure or an interface structurethat has become contaminated with poisons or inhibitors will exhibit ameasurably different electrical response to a change in energy suppliedthereto because of the different thermal properties resulting from thecontamination. In a number of embodiments, interrogations are based onthe measurement of dynamic action of a thermally transitioning structureand its associated electrical signals, which stands in contrast to otherinterrogation methods rooted in static analysis of steady-state signals.A dynamic interrogation pulse (in which applied energy is increased ordecreased over a defined period of time) may be applied to an elementthat is otherwise operating in a continuous mode, whereinenergy/temperature is maintained relative constant in one or more modesthereof, or in pulse-mode or pulse width modulation operation asdescribed below. Like other interrogations methods hereof, dynamicinterrogation measurements may be carried out in the ambient atmosphere(for example, air) without the application of a calibration gas, testgas or other gas. Dynamic interrogation measurements may, for example,be more sensitive to deposition of contaminants than steady-state orcomparative measurements.

A dynamic-mode baseline response may first be established when there ishigh confidence that the element or elements have not been contaminated(for example, may be determined at the time of manufacture). A devicemay subsequently be placed in the dynamic-mode interrogation asdescribed above to determine if contamination (poisoning/inhibition) hasoccurred. One or more threshold values may, for example, be establishedfor slope of the curve, shape of the curve, area under the curve, orvalues at one or more times along the curve. Once again, suchinterrogations may, for example, occur periodically over time. Thecontrol system of the sensor systems hereof may automatically initiatesuch a dynamic-mode interrogation on a periodic or other basis.Moreover, a dynamic-mode interrogation may also be initiated manually.

In the case of dynamic mode interrogation, using an element having arelatively low thermal time constant enables decreasing or minimizingthe length of the dynamic mode interrogation and the power used thereinas compared to an element having a higher thermal time constant. Asdescribed above, the first sensing element may have a thermal constantof 8 second or less, 6 seconds or less, 1 second or less, 500 msec orless, or 250 msec or less.

The nature of the stimulus or interrogation pulse of energy, from anelectrical standpoint, may be a step function or a controlled ramp orcurve from one level to another and (optionally) back again in eitherdirection applied to one or more interface structures of contaminantsensing elements hereof in one or more circuits simultaneously. Thepurpose of the pulse or brief energy change is to cause the changes inthe thermodynamic properties of the interface system (arising from masschanges associated with contamination) to be revealed as it heats orcools. Because the structure is part of sensitive electronic circuitry,for example, including a Wheatstone bridge, simulated Wheatstone bridgeor other bridge/simulated bridge configuration, the electricalproperties of the electronic circuitry are changed in ways that aremeasurably different depending on the thermodynamic response of theelement(s) to the stimulus pulse. These differences can then be analyzedleading to determinations that can be made about the physical conditionof the structure.

Pulse width modulation may, for example, be used to control the energydelivered to elements hereof. Pulse width modulation is a well-knowncontrol technique used to control the average power and/or energydelivered to a load. In embodiments hereof, a voltage is supplied toheat an element to a desired temperature. Because the elements hereofmay have relatively low thermal mass, the cycle times can be relativelyshort.

In pulse width modulation, heating energy (that is, heating voltage(s)or heating currents(s)) may be periodically supplied to the heatingelement(s) during an “ON time”. Rest energy (that is, rest voltage(s) orrest current(s)), which is less than the heating energy may be suppliedduring a “REST time”. The total of the higher-energy or ON time plus thelower-energy or REST time correspond to a cycle time or a cycleduration. Gas concentration or the analyte is measured during the ONtime. The heating energy (voltages/currents) supplied during the ON timemay be constant during the ON time or may be varied (for example,supplied as heating voltage/current plateau or as heatingvoltage/current ramp). The rest energy (voltages/currents) may be equalto zero, or be sufficiently lower than the heating energy so that thegas sensor does not consume any gas or substantially any gas to bedetected. Similar to the ON time, the rest energy supplied during theREST time may be constant during all the REST time or may be varied (forexample, supplied as rest voltage/current plateau or as restvoltage/current ramp). The cycle may be repeated.

An advantage to operating in pulse mode is significantly lower powerconsumption as compared to a continuously powered mode. Anotheradvantage is improved span response as a result of adsorption of excesscombustible gas on the catalyst at cooler temperatures during unpoweredor lower powered operation (that is, during the REST time) as comparedto continuously powering the catalyst at the run temperature of, forexample, 350-600° C.

FIG. 4A sets forth a schematic illustration of a representativeembodiment of a system hereof. In the embodiment of FIG. 4A, a sensordevice, instrument or system 5 includes one or two elements orelement/detector assemblies 110 (a first element/pelement, as describedin connection with FIGS. 2A through 2C) and 110 a (a secondelement/pelement as described in connection with FIGS. 2A through 2C) toform a combustible gas sensor. In FIG. 4A, components of second element110 a are numbered similarly to like components of first element 110,with addition of the designation “a” thereto). First element 110 andsecond element 110 a may, for example, be incorporated within orconnected to electronic circuitry 300 (for example, via or as part of aWheatstone bridge) to measure a concentration of an analyte. In a numberof embodiments, at any time, one of elements 110 and 110 a operates asan analyte element and the other of elements 110 and 110 a operates as acompensating element. In a number of embodiments, each of elements 110and 110 a may include an active catalyst layer and can be alternated infunction as analyte sensing element via temperature control thereof asdisclosed in, for example, U.S. Pat. No. 8,826,721. The function ofanalyte element (high power/high temperature operation) and compensatingelement (for example, low power/low temperature operation) may, forexample, be switched between elements 110 and 110 a on a periodic basis.In other embodiments, one of elements 110 and 110 a includes an activecatalyst layer, while the other of elements 110 and 110 a includes nocatalyst or a deactivated catalyst. A dedicated compensating element mayinclude a deactivation layer (for example, a poison layer) whichdestroys activity thereof to oxidize combustible gas analytes. In such acase, the one of elements 110 and 110 a including the active catalyst isalways operated as an analyte element, while the other of elements 110and 110 a is always operated as a compensating element.

A separate contaminant sensor is provided which includes third, separateelement, a contaminant sensing element 110′. Contaminant sensing element110′ is, in a number of manners, fabricated similarly toelement/detector assembly 110, and components of contaminant sensingelement 110′ are numbered similarly to like components of first element110, with addition of the designation “‘” thereto. Contaminant sensingelement 110’ may, for example, include a catalyst layer, an inactivatedcatalyst layer, or other inactivating layer (as known in the art forcompensating elements), but need not. Interface structure 170′ need onlybe functional or operation to adsorb contaminant thereon and undergomeasurable changes in thermodynamic response properties as a resultthereof. A compensating element for contaminant sensing element 110′ maybe provided. In a number of embodiments, second element 110 a may, forexample, operate as a compensating element for contaminant sensingelement 110′ and compensation for first element 110 may be accomplishedby a temperature transducer (not shown). Second element 110 a may,alternatively, provide compensation for each of first element 110 andcontaminant sensing element 110′. Optionally, a separate compensatingelement 110 a′ (illustrated in dashed lines in FIG. 4A) may be providedfor contaminant sensing element 110′, while second element 110 afunctions only to compensate for first element 110. Compensating element110 a′ may, for example, be matched in manufacture to contaminantsensing element 110′ but may be substantially inactivated to massdeposition as further described below. Components of compensatingelement 110 a′ are numbered similarly to like components of contaminantsensing element 110′, with addition of the designation “a′” thereto.

Electronic circuitry 300 may be, for example, placed in electricalconnection with contact posts 130, 130 a and 130′ of each of assemblies110 via a printed circuit board or PCB (not shown in FIG. 4A). A powersource 304 provides power to electronic circuitry 300. In the case of asensor fixed at a position within a facility, power may be provided froma remote source. In the case of a portable sensor, power source 304 mayinclude one or more batteries. Electronic circuitry of sensor system 5may also include a control system 306 which may, for example, includecontrol circuitry and/or one or more processors 310 (for example, amicroprocessor) and an associated memory system 320 in communicativeconnection with processor(s) 310. A user interface 330 (including, forexample, audible, visual (for example, via a display) or tactileinformation transmission) to provide information to a user and via whicha user may input information (for example, via a keyboard, touchscreenor other input device) and a communication system 340 (for example,including a wired and/or wireless data transceiver for remoteinformation/data transmission) may also be provided. FIG. 4B illustratesan embodiment of a simulated Wheatstone bridge circuit incorporatingcontaminant sensing element 110′ and compensating element 110 a′ whichforms a part or portion of circuitry 300.

In a number of studies, contaminant sensing element 110′ was formedusing the same manufacturing methodologies as that of catalyticallyactive analyte detecting/sensing element to include a catalyst supportedon interface structure 170. However, contaminant sensing element 110′ asincorporated and operated in the system of FIGS. 4A and 4B wasinoperable to determine a concentration of a combustible gas analyte.Interface structure 170′ and interface structure 170 a′ were formed of arefractory composition including aluminum oxide. Interface structure170′ was impregnated with a noble metal catalyst. In a number ofrepresentative studies, it was found that dynamic diagnostics on thecontaminant sensor including contaminant sensing element 110′ andcompensating element 110 a′, when operated at a step voltage of 1.85 V,showed a change in response, compared to the uncontaminated state, at aheating time of 200 ms into a step of −0.87 mV±0.62 mV (mean±standarddeviation) for a dose of 44 ppm-hours hexamethyldisiloxane (HMDS).Alternately, the heating curves can be fit to splines, as known in thecurve fitting arts, which can predict HMDS dose with an R² of 0.91.Additional or alternative data analytical methods known to those skilledin the art such as, for example, area under the heating curves, may beused to predict HMDS dose.

In representative embodiments, a voltage step change of, for example,2.5 seconds on, 10 seconds off, may be repeated several times and alater pulse (for example, the second pulse, the third pulse or a laterpulse) is used for contaminant diagnosis. The third pulse was used in anumber of embodiments hereof. In a number of studies, the period of timebetween contaminant exposure and dynamic diagnosis yielded similarcorrelations for time periods between 1 and 15 minutes.

In a number of studies, the material composition of contaminant sensingelement 110′ was varied by excluding noble metals catalysts frominterface structure 170′. Further, unlike the case of compensatingelements, no catalyst inactivating treatments were applied to therefractory aluminum oxide of interface structure 170′ of contaminantsensing element 110′ hereof. Interface structure 170 a′ likewiseincluded a refractory aluminum oxide and no metal catalyst and wassubstantially inactivated to mass deposition of contaminants asdescribed below. Contaminant sensing element 110′ in such studies thusincluded (or consisted essentially of, or consisted of) heating elementor component 140′ (including helical coil section 142′) covered inmetal-oxide, ceramic interface structure 170′. In a number ofembodiments, of a metal-oxide, ceramic interface structure 170′ wasformed of high surface area aluminum oxide. It was found that dynamicdiagnostics on an oxide-only interface structure 170′ of contaminantsensing element 110′, when operated at a step voltage of 1.85 V, showeda change in response, compared to the uncontaminated state, at a heatingtime of 200 ms into a step of −0.96 mV±0.25 mV for a dose of 44ppm-hours HMDS. Data for such studies are set forth in FIGS. 5A and 5B.Alternately, the heating curves may be fit to splines which can predictHMDS dose with an R² of 0.94, as illustrated in FIG. 6. As describedabove, additional or alternative data analytical methods known to thoseskilled in the art such as, for example, area under the heating curves,can be used to predict HMDS dose.

As also described above, a voltage step change of, for example, 2.5seconds on, 10 seconds off, may be repeated several times and a later(for example, the second, the third or a later pulse) may be used forcontaminant level diagnosis. Once again, the third pulse was used in anumber of studies hereof. The time between poison exposure and dynamicdiagnosis yielded similar correlations for times between 1 and 15minutes. Lower power operation was investigated by lowering the voltagesetpoint on oxide-only contaminant sensing elements 110′. Lower poweroperation (achieved, for example, by lowering the voltage setpoint onthe elements) is possible to conserve energy. The operational power wasnot optimized in the experimental studies of the systems hereof, butsuch optimization is readily achievable for a particular system usingknown engineering principles. An oxide-only contaminant sensing element110′ (that is, without a metal catalyst supported thereof) was selectedfor further study because of its superior statistical predictive poweras compared to use of a standardly produced catalytic analyte sensingelement (including a supported noble metal catalyst) as contaminantsensing element 110′.

To ameliorate the effects of environmental contaminants or poisons,manufacturers of catalytic combustible sensors often incorporatefiltration material(s) upstream of the catalytically active element(s)to trap the contaminating compounds. Such filters may, for example,function on physiochemical processes such as physisorption,chemisorption, chemical reaction, or a combination thereof to increasethe span stability and lifetime of the combustible gas sensor. Filtersfor combustible gas sensors may, for example, include a variety of metalsalts, activated carbon, adsorbent metal oxides or combinations thereofwhich have been found to reduce the effective concentration ofcontaminants reaching the catalytically active analyte (sensing)element. Representative examples of such filters are, for example,disclosed in United States Patent Application Publication No.2018/0353885 and U.S. Pat. No. 6,756,016, the disclosures of which areincorporated herein by reference. Upstream filtration is not limited toseparate or external filters. In that regard, filter materials may becoated directly onto the catalyst-supporting surface of supportstructures such as supports structure 170.

A consequence of including upstream filtration in combustible gassensors is that sensitivity and response time can be reduced forcombustible gas analytes of interest. A reduction in sensitivity orresponse time may be especially troublesome for heavy hydrocarbons whenfilters including adsorbents with high surface area (for example,greater than 75 m²/g) or relatively thick filters are used.

The contaminant sensing elements hereof differ from contaminantdetection or sensing structures/elements in previous combustible gassensor systems in that the contaminant sensing element hereof arephysically separate from the analyte sensing element and anycompensating element. The contaminant sensing elements hereof may thusoperate in a physically different location/environment than the analytesensing element (as well as the compensating element) within the devicesand systems hereof. In a number of embodiments of devices, systems andmethods hereof, the separate contaminant sensing element(s) experience adifferent (for example, a lesser) degree of adsorbent filtration thandoes the analyte sensing element(s). A significant advantage is providedin such embodiments in that a high-magnitude contaminant response ispossible when the contaminant sensing element is exposed to highercontaminant doses than the combustible gas analyte element can withstandwithout total loss of sensitivity.

In a number of representative embodiments, a combustible gas analyteelement in the form of element/pelement 110 exhibited an initialsensitivity of 65 mV per 2.5% vol methane in air (operated in a constantvoltage mode). Studies showed that upon exposure of a continuouslypowered analyte element to 15 ppm HMDS in air for 20 minutes withoutupstream adsorbent filtration, the post-contaminant sensitivity wasreduced to 32 mV in 2.5% vol CH₄. The sensitivity (or the ratio betweenthe output signal and the measure property, or vol % in this case) wasthus reduced to approximately one half of the original non-contaminatedsensitivity. Therefore the “contamination tolerance” or “poisontolerance” of the continuously powered analyte element to its“half-life” is a 5 ppm-hour HMDS dose, wherein ppm-hour is the productof the concentration and the exposure time.

As known to those skilled in the art, the overall or effectivecontaminant/poison tolerance of the above analyte element can beextended beyond a device or instrument experienced dose of 5 ppm-hourHMDS by including an adsorbent material (filter) physically locatedbetween the analyte element and the environment to be tested. In anumber of embodiments, such an adsorbent material may be formed as apressed-powder filter pellet. When sensors including such an upstreamfilter were exposed to HDMS contaminant, the contaminant tolerance(measured as dose to half-life) was determined to be 100-200 ppm-hourHMDS for the continuously powered analyte element (in the form ofpelement 110). In other words, when exposed to 15 ppm HMDS/2.5% volmethane/air, the dose required to reduce the analyte element sensitivityfrom 65 mV to 32 mV was 100-200 ppm-hours HMDS, or an exposure time of6.7-13.3 hours (see FIG. 7). At lower doses, such as 50 ppm-hour HMDS,the analyte element sensitivity was observed as 52±5 mV in the sameexperiment. A 50 ppm-hour HMDS dose upstream of the filter thus resultedin 20% sensitivity loss to the analyte element downstream of the filter.A 50 ppm-hour HMDS exposure for an unfiltered analyte element wouldresult in total span loss. The dose corresponding to 20% span loss on anunfiltered element is 2 ppm-hour.

In a number of embodiments hereof, the transport pathway or filterpathway (that is, the pathway between an element and the environment tobe tested, which may include one or more filters or filter components)is different for an analyte element or elements and a correlatedcontamination sensing element. The contamination sensing element may,for example, be positioned or located so that one or more contaminantfilters (such as one or more adsorbent filters suitable to filtercatalyst poisons such as organosilicon compounds) which are locatedbetween the environment being tested and the analyte element is/are notbetween the environment being tested and the contaminant sensingelement. In such embodiments, the filter capacity of the filter pathwayfor the analyte element is greater than the filter capacity of thefilter pathway for the contaminant sensing element. In a seriesarrangement, the contaminant sensing element may, for example, bepositioned upstream of a particular contaminant filter or filters whilethe analyte element is positioned downstream of the filter or filters.Alternatively, parallel pathways with different filter capacity may beused.

In the example of the filter discussed above, the contaminant sensingelement would experience a dose of 50 ppm-hour HMDS, while the filteredanalyte element experiences only 2 ppm-hour HMDS, resulting in a 20%sensitivity loss in the analyte element. Once again, a significantadvantage provided by the devices, systems and methods hereof is thehigh-magnitude contaminant response possible when the separatecontamination sensing element is exposed to a high dose of contaminant(for example, a 50 ppm-hours HMDS dose in the above example) compared toa low dose of contaminant (for example, the 2 ppm-hours HMDS dose in theabove example) for the combustible analyte element. The combustibleanalyte element can withstand the low dose experienced thereby whileretaining sensitivity (for example, an 80% sensitivity retention in theabove example). A user may thus be alerted to contaminants in theenvironment while the analyte element retains sufficient sensitivity forsafety alerts. In a number of embodiments, a sensor output correctionalgorithm can be implemented wherein the reported response of theanalyte element may be increased in an amount proportional to thepredicted span loss (based upon contamination sensing element output).As clear to those skilled in the art, one may calibrate the response ofthe contaminant sensing element to the contaminant(s) and characterizethe differences in filtration or filtration capacity between the analyteand contaminant sensing to determine a contaminant dose experienced bythe analyte element.

For the measured response from the contamination sensing element to beused to predict or determine the contamination dose to which the sensorand therefore the analyte element was exposed, it must sufficientlysample the environmental contamination dose and interact with thecontaminant to undergo mass addition and, therefore, contaminantresponse. A number of sampling approaches are possible which may, forexample, be varied dependent upon the operation of analyte element. Itmay be desirable in some embodiments to, at least partially, match thetemperature control of a compensating element to the analyte elementwith which the compensating element is correlated. However, it ispossible (for example, via processing) to correlate compensating elementresponse to analyte element contamination when the compensating elementis operated in a different temperature control scheme than the analyteelement.

In one sampling approach, in which the analyte element is operatedcontinuously at a temperature of 350-600° C., the contaminant sensingelement may be operated continuously at a temperature of 350-600° C.,similar to the operation of the analyte element, in between diagnosticmeasurements (for example, dynamic diagnostic measurements). In analternate sampling approach, enabling lower power operation compared toa continuous mode, one may reduce the temperature and/or run time of thecontaminant sensing element. Those skilled in the art recognize that aminimum temperature may be required for oxidation of certaincontaminants on the interface structure of the contaminant sensingelement. Many poisons and/or inhibitors are oxidized on the surface ofan element (for example, on a support structure or interface structurehereof) at a certain minimum temperature, sometimes referred to as“light-off” temperature. In the representative example of siloxanevapor, oxidation of the siloxane vapor on the element occurs below thetemperature required for combustible gas detection on a noble metalcatalyst. HMDS is a common siloxane contaminant and has a relatively lowlight-off temperatures compared to methane. Specifically, the light-offtemperature of HDMS is greater than 150° C. as illustrated in FIG. 8,but well below the light off temperature of hydrocarbons such asmethane. Heating a contaminant sensing element hereof via Joule heatingto a temperature below a light-off temperature in the case of acontaminant such as HDMS may result in desorption of the contaminant andany effect upon thermodynamic response of the element may not bemeasurable. Relatively quickly heating the contaminant sensing elementto a temperature above the light-off temperature results in oxidation ofthe HDMS to a species tightly bound upon the interface structure.Another contaminant, with a different physiochemistry, may becomesufficiently bound to the interface structure to affect thethermodynamic response thereof without oxidation or other reaction onthe surface of the interface structure. However, sufficient energy forJoule heating is required to effect a change in the temperature of acontamination sensing element hereof so that changes in thethermodynamic response of the contamination sensing element may bedetected. In general, any composition that deposits upon the interfacestructure of a contaminant sensing element hereof to increase the massthereof in the Joule heating temperature range can be detected. Suchcompositions include, but are not limited to sulfur compounds,silicon/organosilicon compounds, lead compounds, organophosphatecompounds and halogenated compounds. For example, in a number ofrepresentative studies, it was found that dynamic diagnostics oncontaminant sensing element 110′, when operated at a step voltage of1.85 V, showed a change in response at a heating time of 150milliseconds (ms) into a step of −0.17 mV for a dose of 6 ppm-h hydrogensulfide.

In the case of a contaminant sensor hereof for use in detectingsulfur-containing compounds, one or more sulfur active chemisorptioncompositions such as, for example, tin oxide, zinc oxide, copper oxide,and combinations thereof, may be used to enhance sensitivity of thecontaminant sensor to deposition of sulfur-containing compound (forexample, H₂S). Such contaminant-specific compositions may be added tosensing element 110′ or sensing element interface structure 170′. Asclear to those skilled in the art, other surface chemistries orcompositions may be used to facilitate mass deposition (for example,adsorption/chemisorption) of other contaminant compositions.

With respect to run time of contaminant sensing elements hereof, studiesof devices and systems hereof have shown that sampling the sensor vaporenvironment for virulent contaminants with the contaminant sensingelement at a very low power over the course of 1-15 minutes is asufficient temperature cycling rate (as, for example, illustrated inFIG. 9). Without limitation to any mechanism, it is hypothesized thatcool operation of the contaminant sensing element, wherein contaminationsensing element is operated at very low power, allows adsorption sites(such as oxide sites) of the interface structure of the contaminantsensing element to collect the contaminant under favorable adsorptionproperties (that is, at cool conditions). Under such cool conditions,the contaminant (for example HMDS) is in a condensed or adsorbed statebut remains chemically unaltered. When the contaminant sensing elementis subsequently heated above the light-off temperature of thecontaminant (which can, for example, occur relatively quickly for anelement of lower thermal mass), the contaminant available on theadsorption sites is reacted (generally oxidized in the case ofsilicon-containing contaminants) to a strongly held species. In therepresentative example of HDMS, silicon dioxide or a Si_(x)C_(y)O_(z)species results upon heating. In a number of embodiments, thecontaminant sensing element is heated to approximately 2.4 V for asingle “loading pulse” with a duration of 1000 ms every 5 minutes.Approximately every four hours, the contaminant sensing elementundergoes a dynamic diagnostic, which involves five pulses, preferablyto 2.4 V, lasting 2500 ms each, 10 seconds apart. The power required forthese two operations is about 1 mW, compared to a continuous operationpower draw of 100 mW per element (pelement).

To reduce power consumption in a number of embodiments, the analyteelement may be formed on a low-power MEMS hotplate sensor. MEMS sensorelements or pellistors (as described in connection with FIGS. 2A and 2B)generally have lower thermal mass than low-thermal-mass pelements (asdescribed in connation with FIGS. 2A through 2C). As described above,conventional catalytic combustible gas sensor or detectors (for example,those including relatively large thermal mass pelements) are operated ina Wheatstone bridge circuit in constant current, constant voltage orconstant resistance modes, in which the pelements are powered to run ina 350-600° C. range whenever the sensor is operational. That operationalmode can be termed a “continuous” mode. In an alternative mode,particularly suitable for low-thermal-mass elements (for example,low-thermal-mass pelements or MEMS hotplates), one may quickly heat andcool the element(s) in a reduced power mode. For example, a MEMShotplate may be powered for 1 second, then unpowered for 9 seconds,which can be referred to as operation at 10 second, 10% duty cycle. Anobvious advantage to running in reduced power mode is significantlylower power consumption compared to a continuous mode. Another advantageto operation in such a reduced power mode is improved span responseresulting from adsorption of excess combustible gas on the catalyst atcooler temperatures (during unpowered or low-powered, and thuslow-temperature operation) compared to continuously powering thecatalytically active analyte element at the run temperature of 350-600°C. In a number of embodiments, the MEMS hotplate is powered on 0.35seconds then unpowered for 3.65 seconds for operation at a 4 second,8.75% duty cycle. The power consumption of a MEMS hotplate operated inthat manner is approximately 15 mW. Previously available, continuouslyoperated MEMS hotplates consume approximately 100 mW.

In a number of embodiments, a MEMS hotplate including an analyte elementis positioned such that an adsorbent filter is between the environmentto be tested and the MEMS hotplate. The separate contaminant sensingelement may be incorporated or positioned within the system so that noabsorbent filter is present between the environment to be tested and thecontaminant sensing element. For example, an absorbent filter can bepositioned intermediate or between the MEMS hotplate and the contaminantsensing element so that the MEMS hotplate is downstream from theabsorbent filter and the contaminant sensing element is upstream of theabsorbent filter. The absorbent filter may, for example, be designed tohave a capacity so that the contaminant sensing element has appreciablesignal at a contaminant sensitivity that correlates with the analytesensing MEMS hotplate retaining sufficient analyte sensitivity forsafety alerts. In a number of embodiments, an output correctionalgorithm may be applied wherein the reported response from the analyteelement may be increased proportional to the predicted span loss(determined, at least in part, on the basis of the contamination levelmeasured by the contamination sensing element and the effect of theadsorbent filter pathway of the analyte element in reducing thecontaminant dose experienced by the analyte element).

In a number of embodiments, a primary combustible gas sensor (such as acombustible gas sensor including helical coil-formed elements such aspelements 110 and 110 a) and a trigger sensor (such as a MEMS hotplatesensor 200) may be combined into a single device, system or instrument100 as, for example, illustrated in FIGS. 10A and 10B. In general, alower-powered trigger sensor is used to activate a higher-powered,primary combustible gas sensor which includes one or more elementshaving higher thermal mass than the one or more elements of the triggersensor. Low power MEMS hotplate sensor 200 may, for example, operateregularly at, for example, an 8.75% duty cycle. Higher power pelements110 and 110 a (which are combined in operation to form the primarycombustible gas sensor) may, for example, run in a very low power“standby” state in the absence of combustible gases. Once sensor device100 is exposed to a combustible gas environment, the analyte is detectedby MEMS hotplate (trigger) sensor 200, which subsequently “triggers”higher power analyte pelements 110 and 110 a to power up to an operatingtemperature (for example, in continuous mode). Triggered operation of aprimary combustible gas sensor is, for example, described in U.S. patentapplication Ser. No. 16/037,882, the disclosure of which is incorporatedherein by reference. Triggered operation of higher powered pelements 110and 110 a (the primary combustible gas sensor), as compared to analytesensing solely with MEMS hotplate sensor 200, provides improvedlinearity and stability via the higher mass analyte pelements. In anumber of embodiments, a trigger sensor with a single, low-thermal-masselement may be used. In a number of embodiments, when triggered, analytesensing pelement 110/110 a operates in a constant resistance mode, whichprovides better stability over temperature and better analytesensitivity compared to other operating modes.

As, for example, illustrated in FIG. 10A, MEMS hotplate 200, each ofanalyte sensing/compensating pelements 110 and 110 a, and contaminantsensing element 110′ may be placed in connection with electroniccircuitry 300 via a PCB 400. Because system 100 includes at least onecontainment sensing element 110′, which is separate from all analytedetection or sensing elements, separate filter pathways (with differentfiltration characteristics) may be designed therefor. As illustrates inFIGS. 10A and 10B, a matched compensating element for contaminantsensing element 110′ may be absent. Temperature compensation forcontaminant sensing element 110′ may, for example, be accomplished by atemperature transducer (not shown), by compensating element 110 a, or bya combination of the two. As, for example, schematically illustrated inFIG. 10A, a first filter pathway 260 is present between elements 250 and250′ of MEMS hotplate (trigger) sensor 200 and an inlet 104 in housing102 of device 100, a second filter pathway 270 is present betweenpelements 110 and 110 a and inlet 104, and a third filter pathway 280 ispresent between contaminant sensing element 110′ and inlet 104. Each offilter pathways 260, 270 and 280 may include one or more separate filtercomponents and/or one or more filter components may be shared betweendifferent filter pathways. In FIG. 10A, filter pathway 260 includes afirst sorbent filter 262, a second sorbent filter 264 and a sulfurfilter 266, filter pathway 270 includes a first sorbent filter 272 and asulfur filter 274, and filter pathway 280 includes only a sulfur filter282.

In the embodiment of FIG. 11, sorbent filter 262 is present only infilter pathway 260, while sorbent filter 264 is shared between filterpathways 260 and 270, and sulfur filter 266 is shared between filterpathways 260, 270 and 280. Referring to, for example, FIG. 11, onehomogeneous sorbent pellet 264 may or may not be located upstream (thatis, between an environment to be tested and the element) of analyteelement 110 and MEMS hotplate elements 250 and 250′ (that is, withinfilter pathways 260 and 270). Another homogeneous sorbent pellet 262,which may be formed with the same material(s) of sorbent pellet 264, islocated upstream of only MEMS hotplate 200 (that is, in filter pathway270 only). In the embodiment of FIG. 11, sulfur filter 266 is locatedupstream of each of MEMS hotplate sensor element 250 and 250′, analyteelement/element 110, and contaminant sensing element 110′ (that is,within each of filter pathways 260, 270 and 280. The filter pathwaydesigns of FIGS. 10A and 10C provide for additional adsorbent filtration(which may, for example, remove poisons such as HDMS) via filter pathway260 as compared to filter pathway 270. The additional absorbentfiltration of filter pathway 260 provides additional environmentalcontaminant/poison tolerance to lower-mass-elements 250 and 250′ of MEMShotplate sensor 200. On the other hand, analyte element/pelement 110includes less sorbent filtration via filter pathway 270, which allowsfor faster analyte response (once triggered) compared to the morefiltered MEMS hotplate sensor 200. Such multiple-filter-pathways designsmay, for example, speed response to heavy hydrocarbons while stillproviding some contaminant filtration for analyte element 110. Furtherin the embodiment of FIG. 10A, filter pathway 270 is common to bothelements 110 and 110 a, while in the embodiment of FIG. 11, filterpathway 270 is in fluid connection with only element 110, while filterpathway 280 is in fluid connection with contaminant sensing element 110′and compensating element 110 a′ therefor.

In a number of embodiments, each of elements 250 and 250′ of MEMShotplate sensor 100 may or may not include an active catalyst layer andcan be alternated in function as analyte sensing element via temperaturecontrol thereof as disclosed in, for example, U.S. Pat. No. 8,826,721.The function of analyte sensing element (high power/high temperatureoperation) and compensating element (low power/low temperatureoperation) may, for example, be switched between elements 250 and 250′on a periodic basis (for example, every seven days. Sensitivitycorrection based upon measurement of contaminant level via contaminantsensing element 110′ is more complicated in the case of alternating thefunction of elements 250 and 250′. In such embodiments, it may, forexample, be desirable to provide only safety alert based upon measurecontaminant/poison exposure to avoid complexity in processing. It isalso possible, that one of elements 250 and 250′ includes an activecatalyst layer, while the other of elements 250 and 250′ includes nocatalyst or a deactivated catalyst. In such a case, the one of elements250 and 250′ including the active catalyst would always be operated asan analyte sensing element while the other of elements 250 and 250′would be operated as a compensating element.

Like elements 250 and 250′ of MEMS hotplate sensor 200,elements/pelements 110 and 110 a, may each include an active catalystlayer and can be alternated in function as analyte sensing element viatemperature control thereof. In such embodiments, the function ofanalyte sensing element (high power/high temperature operation) andcompensating element (low power/low temperature operation) may, forexample, be switched between elements/pelements 110 and 110 a on aperiodic basis. Alternatively, one of elements 110 and 110 a may includean active catalyst layer, while the other of elements 110 and 110 aincludes no catalyst, a deactivated catalyst, and/or a deactivatedlayer. In such a case, the one of elements 110 and 110 a including anactive catalyst layer would always be operated as an analyte sensingelement while the other of elements 110 a and 110 a would be operated asa compensating element.

In a number of embodiments including MEMS hotplate sensor 200 as a“sniffer” sensor to determine presence of an analyte in an environmentand higher-thermal-mass pelements 110/110 a, which may be triggered” bya positive response from MEMS hotplate sniffer sensor 200, the responseof contamination sensor element 110′ is correlated with the contaminantdose sampled by the regularly cycling MEMS hotplate sensor 200. In thatregard, the MEMS hotplate sensor may be correlated with the measuredcontaminant dose because operation of the MEMS hotplate sensor enablesthe overall sensor analyte gas detection. As for the analyte pelement(such as analyte pelement 110), when it is not powered, it may notexperience significant poisoning resulting from temperature activateddeposition reactions. Because the analyte pelement is powered onlysporadically (that is, only when triggered), additional comparativecalculations would be required to determine the contaminant dose beforeand during operation. In that regard, only high-temperature operationmight measurably load certain contaminants on the analyte detectingpelement, while incident doses during long unpowered or low-powered(that is, low-temperature) operational times should be discounted. Thedegree of contaminant response resolution and the computational effortrequired to track the analyte pelement state during contaminant dosingmay add complexity to the design. Therefore, in a number of embodimentsincluding, for example, goal of analyte sensitivity correction, theresponse from the contaminant sensing element may be correlated withonly the MEMS hotplate sensor analyte sensing element(s).

In other embodiments, including, for example, a goal of analytesensitivity correction, “triggered” analyte pelement 110, in, forexample, the configuration of FIG. 10A through 11, may be operated inthe low-power analyte sampling mode to load contaminants at the samerate as MEMS hotplate sensor and correlated contaminant sensing element110′. Analyte pelement 110 may, for example, operated with a “loadingpulse” to 2.4 V with a duration of 1000 ms every 5 minutes.

In the embodiment illustrated in FIG. 12, a second contaminant sensingelement 110″ is included (in filter pathway 280). Second contaminantsensing element 110″ may, for example, be powered to sample the testedenvironment for contaminants for times representative of the operationtimes of the triggered analyte pelement 110. Although the embodiment ofFIG. 12 requires an additional element compared to the embodiments ofFIGS. 10A through 11, additional contaminant sensing element 110″requires less power to operate than first contaminant sensing element110′, and, like analyte sensing element 110, may remain in a very lowpower state other than when analyte sensing element 110 is triggered.The response of additional or second contaminant sensing element 110″may be readily correlated with analyte pelement 110 with a goal ofanalyte sensitivity correction. Alternately, element 110″ could consistof different materials and/or contaminant sensing regimes in order todetect a different poison than 110 a″.

In embodiment in which the MEMS hotplate sensor includes twocatalytically active elements, the contamination sensing element mayreasonably be correlated with both of the catalytically active elementfor embodiments including the goal of analyte sensitivity correction. Insuch embodiments, both MEMS elements should sample the same contaminantenvironment. This may be accomplished by a configuration that alternatesbetween the element of the MEMS hotplate sensor as the high-temperature,combustible gas analyte sensing element.

Temperature compensation is required for both analyte sensing responseand contaminant sensing response, since both sensors are thermal-basedsensors. In a number of embodiments (see, for example, FIG. 11),combustible analyte sensor temperature compensation may be accomplishedby a temperature transducer (not shown), a low-power, low-temperatureMEMS hotplate element, or a combination of the two. A motivation fordesigning a MEMS hotplate sensor or a low-thermal mass pelement sensorwith two catalytically active elements or detectors is to double thesensor life compared to one active element/detector. As discussed above,the element that is not powered as the analyte sensing element can bepowered to a lower level to function as the temperature compensator asdescribed in U.S. Pat. No. 8,826,721.

Temperature compensation for contaminant sensing elements hereof may,for example, be accomplished using a helical-wire compensator pelement110 a′ which has a sensitivity to mass deposition that is substantiallyreduced via a chemical deactivation process as disclosed in U.S. Pat.No. 5,401,470. Compensator pelement 110 a′ may, for example, be operatedin the same dynamic diagnostic mode as the contaminant sensing element.It was discovered that loading a compensating element for a contaminantelement hereof with, for example, a silicon or organosilicon compoundsuch as HDMS rendered the thermodynamic response of such a compensatingelement substantially insensitive to further mass loading from acontaminant compound. In the case of low-thermal-mass pelements asdescribed above, a dose of approximately 25,000 ppm-h was used to lowerthe sensitivity or the compensating element to mass deposition ofcontaminants.

Because mass deposition of a siloxane compound such as HDMS isdestructive to the sensitivity of contaminant elements hereof to massdeposition, a specific contaminant element cannot be readily calibratedvia exposure thereof to a particular dose of HDMS. By carefulmanufacture of contaminant elements hereof, one contaminant element canbe exposed to, for example, HDMS to determine a calibration for other,like contaminant elements which are manufactured in the same manner.Alternatively, a contaminant/composition which does not form anirreversible bond with the interface structure may be used to calibratea specific contaminant element that may later be used in a contaminantsensor hereof. In that regard, after the calibration, the removablecontaminant may be removed from the contaminant element. For example, asulfur compound may be used to calibrate a particular contaminantelement and subsequently “burned off” that contaminant element at hightemperature.

Using a thermally matched temperature compensating element orcompensator for determination of contaminant exposure may, for example,provide an improved signal-to-noise ratio when compared to a coolelement. This may be particularly advantageous in the case of therelatively small signals generated in determining thermodynamic changesresulting from, for example, mass changes arising from dosages inrelatively low ppm-hour ranges. During field operation, the temperaturemeasured by the sensor temperature transducer may reference theappropriate bridge coefficients to obtain specified contaminationdetection performance. In a number of embodiments, the contaminantsensing element(s) and temperature compensation element(s)/pelement(s)therefor may be positioned in similar but separate thermal environmentswith the same degree of adsorbent filtration or lack thereof. In anumber of embodiments, the contaminant sensing element(s)/pelement(s)and the temperature compensation element(s)/pelement(s) are locateddownstream of only minimal sorbent filtration, or downstream of nosorbent filtration.

In FIG. 11 compensating element 110 a′ operates to compensate forcontaminant sensing element 110′. Compensating element 110 a′ may, forexample, be operated under the same power scheme as contaminant sensingelement 110′ in the embodiment of FIG. 11. In FIG. 12 compensatingelement 110 a″ is operated to compensate for each of contaminant sensingelements 110′ and 110″. Compensating element 110 a″ may, for example, beoperated under a power scheme that is a combination or overlay of thepower schemes if contaminant sensing element 110′ and contaminantsensing elements 110″ in the embodiment of FIG. 12.

Although certain advantages may be achieved using elements having lowvolume/low thermal mass as described above, the devices, systems andmethods described above may also be used with element of relative highvolume/high thermal mass. For example, standard pelements, which mayhave an effective diameter of greater than or equal to 1 mm may be usedherein.

In a number of embodiments, the catalytically active analyte sensingelements may function as auxiliary contaminant sensing elements. Theoperation of catalytically active, sensing or analyte elements to detecta deposited contaminant thereon in both comparative/continuous anddynamic diagnostic modes is disclosed in US Patent ApplicationPublication Nos. 2018/0335412 and 2018/0335411, the disclosures of whichare incorporated herein by reference. In the devices systems and methodshereof, the low level contaminant exposures measurements provided by theanalyte sensing element(s) may, for example, indicate or confirm theneed for a safety alert regarding contaminant penetration of a filter orfilter pathway. Use of the separate contaminant sensing element (thatis, a contaminant sensing element separate from any analyte sensingelement) provides significantly superior contaminant response comparedto use of an analyte sensing element or element to measurecontamination. The improved response enables, for example, improvedinstrument output correction algorithms.

As described in US Patent Application Publication Nos. 2018/0335412 and2018/0335411, an active, sensing or analyte element in a number ofcombustible gas sensors hereof may, for example, be operated at agenerally constant voltage, a constant current or a constant resistance(and thereby at a constant temperature) during a particular mode ofoperation. In a number of embodiments of combustible gas sensors hereof,the electronic circuitry of the combustible gas sensor operates in afirst mode in which a first or sensing element is heated to or operatedat a temperature at which the first catalyst catalyzes combustion of theanalyte gas (for example, above 300° C. for methane). In a second mode,the electronic circuitry operates to heat the sensing element to asecond temperature which is lower than the first temperature. The secondtemperature is below the temperature at which the first catalystcatalyzes combustion of the analyte gas but is at or above a temperatureat which Joule heating of the first element occurs. The secondtemperature may also be below the light off temperature of othercombustible gasses that may be in the environment being tested by thesensor. The second temperature is also typically lower than atemperature at which one or more predetermined inhibitors and/or poisonswhich may be predetermined (for example, inhibitor(s) or poison(s) thatmay be present in the ambient environment) are, for example, oxidizedupon or within the support structure of the first element. Once again,however, the second temperature is at or above the temperature at whichJoule heating occurs so that changes in mass affect upon thethermodynamic properties of the contamination sensing element may bemeasured. In that regard, mass deposition on the surface of all elementshereof changes the thermodynamic response of the elements. Although thechange in thermodynamic response may be measured as an electricalresponse in, for example, a Wheatstone bridge circuit, heating isrequired to observe the response.

The electronic circuitry hereof measures a variable in the second moderelated to a mass of the first element. The variable is measured overtime (that is, through multiple cycles between the first mode and thesecond mode), and change in the variable over time is analyzed to relatethe change in the variable to a change in mass of the first element. Thechange in mass is an indication of deposition of a poison or inhibitorof the catalyst of the first element. For example, voltage, current orresistance of the second element can be measured (depending upon themanner in which the system is driven to control voltage, current and/orresistance in the second mode).

In a dynamic diagnostic scheme, the electronic circuitry is configuredto apply an interrogation pulse to the analyte element in which energyto analyte first element is increased or decreased to induce anassociated response from the analyte element. The electronic circuitryis also configured to analyze the associated response and to determinefrom the associated response if poisoning or inhibiting of the firstcatalyst has occurred. In that regard, one or more thresholds forchanges in response or changes in values may, for example, beestablished which are predetermined to indicate if a change in mass ofan analyte element has occurred. For example, thresholds for changes inresponse such as change in slope of a curve, changes in area under thecurve, and/or changes in values at one or more times along the curve maybe predetermined.

The shape of the response is the result of the associated electroniccircuitry's (for example, a bridge's) response to the non-linear changesin the resistance of the elements. Over the duration of the energypulse, elements are changing from one thermal state to another asdescribed above. The elements do not necessarily change at the same rateat the same point in time during the changing thermodynamic phases ofthe event. The resistance in each element changes (for example,perturbing the balance of the bridge circuit) in step with thenon-linear thermal changes in the heating element and thecatalyst/support structure system. The resulting non-linear change inthe measured variable (for example, voltage) may be referred to as aninterrogation pulse which can be analyzed electronically ormathematically. In addition to various bridge and other circuits, theanalyte element and compensating element may be driven separately.

Elements hereof may transition through three phases during a dynamicdiagnostic energy pulse. In an energy pulse in which the element beginsin a relatively low-energy state (for example, at ambient temperature ora temperature below which joule heating of the heating element occurs),the applied pulse of energy causes dynamic heating of the element. Oneskilled in the art will appreciate that similar information can beobtained from an element that is initially at a high temperature state(for example, an analyte element operating at a temperature at or abovewhich catalytic combustion of an analyte occurs) and energy is removedfrom the element to cause dynamic cooling of the element to a lowertemperature (for example, to a temperature below the temperature atwhich joule heating occurs or to ambient temperature). During joule orresistive heating, passage of an electric current through conductiveheating element releases heat, which may be referred to as a resistivephase. During a conductive phase, heat from the heating elementtransfers from the heating element to the catalyst support structure andthe catalyst supported thereon (conduction or conductive heating). Heattransfer then occurs via fluidic convection (convection or convectiveheating) through the surrounding gases. Eventually, a thermalequilibrium will be reached. Once again, thermal equilibrium will bereached and remain balanced until (a) the ambient temperature changes,or (b) the makeup of the surrounding gas mixture is altered, or (c) thetransfer of heat between the wire and the mass of the element changes(as a result of a mass or density change), all of which are competingand interacting effects.

A response curve hereof may also be obtained in which energy (andcorrespondingly temperature) is decreased from a higher energy state toa lower energy state. In such an embodiment, an element may begin in aconvective phase and transfer through a conductive phase above untilthermal equilibrium is achieved as described above. The decrease inenergy may, for example, be of sufficient magnitude and length such thatthe temperature of the element decreases to a temperature below thetemperature at which Joule heating commences. Differences between thefirst pulse and subsequent pulses have been observed to correlate withthe presence of volatile species, such as water vapor, and ambienttemperature.

In the case of operation of either a contaminant sensing element or ananalyte sensing element hereof to detect mass deposition of one or morecontaminant compositions, additional information may be obtained byexamining the response in the different phases of heating as describedabove. In that regard, the greatest effect from contamination may occurduring the peak conductive heating phase with measurably less or noeffect in the trailing convective phase. This result indicates that theinterface structure or support structure underwent physical changes inits internal structures. For example, this occurs when asulfur-containing contaminant reversibly adsorbs onto the structure. Ifsuch an adsorbate has been identified, one may attempt ahigher-temperature heating period to desorb the contaminant from theelement and return the element to its original sensitivity.

Additional consideration may also be given to the convective phases ofthe interrogation pulses. If significant displacement has occurred inthe trailing convective phase it may indicate that a contaminantmaterial is deposited (for example, oxidized) on the outside of theinterface/support structure, thereby changing the convective heattransfer characteristics. As additional mass deposition occurs, thechange in signal continues to progress and may be represented in manymeasurable forms. Such a result is observed in the case ofsilicon-containing compositions such as HDMS which cannot be removed viahigh-temperature heating. Thus, examining different regions of theresponse curve to a dynamic energy change may provide additionalinformation regarding the nature of the contamination and determinefuture actions to be taken.

The devices, systems and methods hereof may, for example, be used inconnection with other devices, systems and methodologies for detectingcontamination (poisoning or inhibiting) of catalytically active analytesensing elements (including for example, electronic interrogationsmethodologies which do not require application of a test or other gas tothe sensor). For example, devices, systems and methods disclosed in U.S.Patent Application Publication No. 2014/0273,263, the disclosure ofwhich is incorporated herein by reference) may be used. In such devices,systems and methods, a variable related to the complex component ofimpedance, which is sometimes referred to as reactance, of the firstsensing element (variables that may be measured include, but are notlimited to, impedance, reactance, resonant frequency, a frequencydependent variable, inductance, capacitance, or the resistive componentsof inductance and/or capacitance). Changes in the measured variable overtime are used to determine the operational status of the analyte sensingelement. Changes in a variable related to reactance are particularlysensitive to contamination of the interior structure of a catalystsupport structure and may, for example, be used in conjunction withother systems and methods hereof to assist in determining the existenceand nature of any contamination of an element hereof.

In a device, system or method hereof, the measured variable ofcontaminant sensing element hereof may be used to correct gasconcentration output/readings in real-time. Below is a representativeexample of a formula for adjusting the sensitivity of the system.

S _(t) =S _(o)*(D _(o) /D _(t) *k)

In the above equation, S_(t) is the sensitivity at a given time t; S_(o)is the initial or previously determined sensitivity, D_(o) is theinitial or previously determined variable related to the dynamicinterrogation mode, D_(t) is the variable measured at a given time t andk is a scaling factor constant. A lookup table may, for example,alternatively be used to relate a change in the measured variable to asensitivity correction.

Furthermore, one or more measured variables hereof may be used as atrigger to apply additional heat to the catalyst support structure of ananalyte element to potentially remove inhibitors. Periodic measurementof the variable, analysis of the results thereof, correction of sensoroutput and/or application of additional heat may, for example, beeffected by control system 306 (via, for example, an algorithm oralgorithms stored in memory system 320 as software) in an automatedmanner without user intervention. The measurement of a variable (forexample, voltage, current or resistance) and associated application ofadditional heat may be done in real time and offer not only a life andhealth aspect for the system, but a self-curing attribute. Moreover, ifthe sensor fails to “burn off” a contaminant, it can be determined thatthe contaminant is a poison. The user may be notified that the activeelement of the system has been poisoned (for example, via display system340, alarm system of user interface system 330 and/or other userinterfaces). The “burn off” procedure described herein may, for example,be used in connection with any electronic interrogation of the activesensing element that is suitable to determine that a foreign materialhas contaminated the active sensing element.

An electronic interrogation or control algorithm or process may beimplemented as described in US Patent Application Publication Nos.2018/0335412 and 2018/0335411. In that regard, each time a variablerelated or indicative to mass change in a contaminant sensing element ismeasured, it is evaluated. If the variable and/or a correction ofanalyte element sensitivity associated therewith is within normal limits(for example, +/−1% of a predetermined or threshold value), nocorrections occur and the sequence repeats. If a non-conforming resultis obtained (that is, the variable and/or correction is not withinnormal limits), different actions are taken depending upon whethersensitivity should be increased or decreased, which is dependent uponthe measured variable. If the measured variable results in a need toincrease the sensitivity (for example, associated with contamination ofthe sensing element), the algorithm will determine if the increase iswithin normal limits, and do so. If the increase is within normallimits, the system will attempt to increase the heat to burn off anyinhibitors, and the user may, for example, be alerted that this“burn-off” or cleaning process is taking place. If the maximum thermallimit has already been applied, and the maximum correction has also beenapplied, then the user may, for example, be alerted that the analyteelement has been poisoned. If the measured variable results in the needto decrease the sensitivity, the algorithm will determine if thedecrease is within normal limits, and do so. If the decrease is withinnormal limits, the system will check to see if heat had been previouslyapplied to attempt to burn off an inhibitor. If heat had been applied,the heat will be reduced. This control algorithm or a similar algorithmhereof may, for example, be an automated procedure carried out viacontrol system 306 without the need for user intervention. The controlalgorithm may, for example, be embodied in software stored within memorysystem 320 and executed by processor(s) 310 of control system 306. Thecombustible gas sensor hereof may be operative to detect the combustiblegas analyte during the execution of the electronic interrogation,control algorithm or process.

The devices, systems and/or methods described herein can be used inconnection with a variety of types of combustible gas sensors. Existingcombustible gas sensors designs are readily modified to include a deviceor system hereof for measuring a variable related to mass change of oneor more sensing elements thereof. For example, such devices, systemsand/or methods can be used in connection with Micro-Electro-MechanicalSystems (MEMS), thin/thick film system, or other suitable micro- ornanotechnology systems such as, for example, described in U.S. Pat. Nos.5,599,584 and/or 6,705,152, as well as metal oxide semiconductor (MOS)sensors (such as H₂S-MOS sensors and solid state O₂-MOS sensors).

The foregoing description and accompanying drawings set forthembodiments at the present time. Various modifications, additions andalternative designs will, of course, become apparent to those skilled inthe art in light of the foregoing teachings without departing from thescope hereof, which is indicated by the following claims rather than bythe foregoing description. All changes and variations that fall withinthe meaning and range of equivalency of the claims are to be embracedwithin their scope.

What is claimed is:
 1. A system for detecting an analyte gas in anenvironment, comprising: a first gas sensor, a first contaminant sensorseparate and spaced from the first gas sensor, and electronic circuitryin electrical connection with the first gas sensor to determine if theanalyte gas is present based on a response of the first gas sensor, theelectronic circuitry further being in electrical connection with thefirst contaminant sensor to measure a response of the first contaminantsensor over time, the measured response of the first contaminant sensorvarying with an amount of one or more contaminants to which the systemhas been exposed in the environment over time.
 2. The system of claim 1wherein the first gas sensor is a first combustible gas sensor.
 3. Thesystem of claim 2 wherein the first contaminant sensor comprises a firstcontaminant sensor element separate and spaced from the firstcombustible gas sensor, the first contaminant sensor element comprisinga first electrically conductive heating component and a first interfacestructure on the first electrically conductive heating component,wherein the electronic circuitry is configured to provide energy to thefirst electrically conductive heating component, wherein the measuredresponse is a thermodynamic response of the first contaminant sensorelement which varies with mass of the one or more contaminants depositedon the first interface structure thereof.
 4. The system of claim 3wherein the first combustible gas sensor comprises a first elementcomprising a first electrically conductive heating element, a firstsupport structure on the first electrically conductive heating elementand a first catalyst supported on the first support structure, theelectronic circuitry being configured to provide energy to the firstelectrically conductive heating element to heat the first element to atleast a first temperature at which the first catalyst catalyzescombustion of the analyte gas and to determine if the analyte gas ispresent based on the response of the first combustible gas sensor whilethe first element is heated to at least the first temperature.
 5. Thesystem of claim 4 wherein the first contaminant sensor further comprisesa second contaminant sensor element, the second contaminant sensorelement comprising a second electrically conductive heating componentand a second interface structure on the second electrically conductiveheating component, the electronic circuitry being configured to operatethe second contaminant sensor element as a compensating element for atleast the first contaminant sensor element to compensate for ambientconditions.
 6. The system of claim 5 wherein the second contaminantsensor element is treated to be generally insensitive to at least one ofthe one or more contaminants.
 7. The system of claim 6 wherein thesecond contaminant sensor element is treated with a predetermined amountof an oxidized organosilicon compound.
 8. The system of claim 5 whereinthe first interface structure is selected to adsorb at least one of theone or more contaminants that undergo oxidation upon heating.
 9. Thesystem of claim 5 wherein the first interface structure comprises anoxide.
 10. The system of claim 9 wherein the first interface structurehas a surface area of at least 75 m²/g.
 11. The system of claim 5wherein the first contaminant sensor element comprises no metalcatalyst.
 12. The system of claim 5 wherein the first contaminant sensorelement consists essentially of the first electrically conductiveheating component and the first interface structure, which consistsessentially of an oxide.
 13. The system of claim 1 further comprising afirst filter pathway between the first gas sensor and the environment,the first filter pathway having a first capacity to remove at least oneof the one or more contaminants, and a second filter pathway between thefirst contaminant sensor and the environment, the second filter pathwayhave a second capacity to remove at least one of the one or morecontaminants, wherein the second capacity is less than the firstcapacity.
 14. The system of claim 13 wherein the first capacity comprisea first adsorbent filtration capacity and the second capacity comprisesa second adsorbent filtration capacity, less than the first adsorbentfiltration capacity.
 15. The system of claim 5 further comprising afirst filter pathway between the first element of the first combustiblegas sensor and the environment, the first filter pathway having a firstcapacity to remove at least one of the one or more contaminants, and asecond filter pathway between the first contaminant sensor element andthe environment, the second filter pathway have a second capacity toremove at least one of the one or more contaminants, wherein the secondcapacity is less than the first capacity.
 16. The system of claim 15wherein the first capacity comprise a first adsorbent filtrationcapacity and the second capacity comprises a second adsorbent filtrationcapacity, less than the first adsorbent filtration capacity.
 17. Thesystem of claim 16 wherein the second adsorbent filtration capacity iszero.
 18. The system of claim 15 wherein the first contaminant sensorelement is low-thermal mass element.
 19. The system of claim 18 whereinthe first contaminant sensor element has a thermal time constant lessthan 8 seconds.
 20. The system of claim 5 wherein a pulse is applied tothe first contaminant sensor element in which energy to the firstcontaminant sensor element is increased or decreased to induce themeasured response from the first contaminant sensor element, theelectronic circuitry being configured to analyze the measured response.21. The system of claim 20 wherein the electronic circuitry isconfigured to apply a plurality of pulses to the first contaminantsensor element over time in which energy to the first element isincreased or decreased to induce the measured response from the firstcontaminant sensor element in each of the plurality of pulses, theelectronic circuitry being configured to analyze one or more of themeasured responses.
 22. A method for detecting an analyte gas in anenvironment, comprising: providing a first gas sensor, providing a firstcontaminant sensor separate and spaced from the first gas sensor,providing electronic circuitry in electrical connection with the firstgas sensor and with the first contaminant sensor, measuring a responseof the first gas sensor to determine via the electronic circuitry if theanalyte gas is present, and measuring a response of the firstcontaminant sensor to determine via the electronic circuitry if the gassensor has been exposed to one or more contaminants, wherein themeasured response of the first contaminant sensor varies with an amountof one or more contaminants to which the system has been exposed in theenvironment over time.
 23. The method of claim 22 wherein the first gassensor is a first combustible gas sensor.
 24. The method of claim 23wherein the first contaminant sensor comprises a first contaminantsensor element separate and spaced from the first combustible gassensor, the first contaminant sensor element comprising a firstelectrically conductive heating component and a first interfacestructure on the first electrically conductive heating component,wherein the electronic circuitry is configured to provide energy to thefirst electrically conductive heating component, and wherein themeasured response of the first contaminant sensor is a thermodynamicresponse of the first contaminant sensor element which varies with massof the one or more contaminants deposited on the first interfacestructure thereof.
 25. The method of claim 24 wherein the firstcontaminant sensor further comprises a second contaminant sensorelement, the second contaminant sensor element comprising a secondelectrically conductive heating component and a second interfacestructure on the second heating electrically conductive heatingcomponent, the method further comprising operating the secondcontaminant sensor element via the electronic circuitry as acompensating element for at least the first contaminant sensor elementto compensate for ambient conditions.
 26. A system, comprising:electronic circuitry comprising a control system; a primary combustiblegas sensor in electrical connection with the electronic circuitry todetermine if an analyte gas is present based on a response of theprimary combustible gas sensor; a trigger combustible gas sensor inelectrical connection with the electronic circuitry to determine if theanalyte gas is present based on a response of the trigger combustiblegas sensor, wherein the electronic circuitry is configured to operatethe trigger combustible gas sensor to detect a value of the response ator above a threshold value, the primary combustible gas sensor beingactivated from a low-power state upon the threshold value being detectedby the trigger combustible gas sensor; and a first contaminant sensor inelectrical connection with the electronic circuitry and being positionedseparate and spaced from the primary combustible gas sensor and from thetrigger combustible gas sensor, the electronic circuitry further beingconfigured to measure a response of the first contaminant sensor overtime, the measured response of the first contaminant sensor varying withan amount of one or more contaminants to which the system has beenexposed in the environment over time.